After having some hard times with days 32 to 34 and several repetitons of those days, it's done.

I could finally finish the milestone project of the educational app and it took me about 10 hours... But I must say that I learned more of it, than in any milestone and challenge before. Time to move on with a bunch of self-esteem. If someone has suggestion, I'm open for all type of criticism.

Thanks to Paul, who is motivating me each day :D

//
//  ContentView.swift
//  Plutimication

import SwiftUI

struct Background: View{
    var body: some View {
        ZStack {
            RadialGradient(gradient: Gradient(colors: [.yellow,.brown, .blue ]),
                           center: .init(x: 0.5, y: 0.50),
                           startRadius: 20, endRadius: 280)

            VStack{
                Text("\nPlutimication")
                    .padding(20)
                    .font(.largeTitle.weight(.black))
                    .shadow(color: .gray, radius: 4, x: 5, y: 5)
                    .opacity(0.7)

                Spacer()
            }
        }
        .ignoresSafeArea()
    }
}

struct ContentView: View {

    @State private var difficulty: Int = 5
    @State private var numberOfQuestions = 5

    @State private var optNumberOfQuestions = [ 5, 10, 20]
    @State private var rangeDifficulty = 3...12

    @State private var questions = [(num1: Int, num2: Int)]()
    @State private var answers = [Int]()

    @State private var round = 0
    @State private var score = 0

    @State private var showMenuButtons = true
    @State private var showParameters = false
    @State private var gameHasStarted = false
    @State private var gameOver = false

    var body: some View {
        ZStack {
            Background()

            // Startmenu view
            if showMenuButtons {
                VStack{
                    Spacer()
                    Button ("Start Game") {
                        withAnimation {
                            startGame()
                        }
                    }
                    .padding(60)
                    .background(.green.opacity(0.4))
                    .clipShape(Circle())
                    .transition(.scale)

                    Button () {
                        withAnimation {
                            setParameters()
                        }
                    } label: { Image(systemName: "gearshape").resizable().frame(width: 35, height: 35) }
                        .padding(15)
                        .background(.blue.opacity(0.4))
                        .foregroundColor(.yellow.opacity(0.6))
                        .clipShape(Circle())
                        .transition(.scale)

                    Spacer()
                }
                .transition(.scale)
            }

            // Parameter view
            if showParameters {
                VStack(spacing: 30) {
                    Section{
                        Picker ("Number of Questions:", selection: $numberOfQuestions) {
                            ForEach (optNumberOfQuestions, id: \.self) {
                                Text("\($0)")
                            }
                        }
                        .pickerStyle(.segmented)
                    } header: { Text ("Choose number of questions:") }

                    Section{
                        Picker ("Difficulty:" , selection: $difficulty) {
                            ForEach (rangeDifficulty, id: \.self) {
                                Text("\($0)")
                            }
                        }
                        .pickerStyle(.segmented)
                    } header: { Text ("Choose level of difficulty:") }

                    Button("Done"){
                        withAnimation{
                            showParameters = false
                            showMenuButtons = true
                        }
                    }
                    .padding(10)
                    .background(.blue.opacity(0.4))
                    .foregroundColor(.yellow.opacity(0.6))
                    .clipShape(Capsule())
                }
                .frame(width: 260, height: 300, alignment: .center)
                .padding(20)
                .background(.white.opacity(0.2))
                .foregroundColor(.black.opacity(0.6))
                .clipShape(RoundedRectangle(cornerRadius: 30))
                .transition(.scale)
            }

            // game is started
            if gameHasStarted && !gameOver {
                VStack(spacing: 20){
                    Text ("Round \(round+1) of \(numberOfQuestions)")

                    Text ("How much is \(questions[round].num1) * \(questions[round].num2)?")
                        .font(.headline)

                    HStack{
                        ForEach (answers, id: \.self) {num in
                            Button (){
                                withAnimation{
                                    buttonTapped(num)
                                    nextRound()
                                }
                            } label: { Text ("\(num)") }
                                .padding(12)
                                .foregroundColor(.yellow)
                                .background(.blue.opacity(0.5))
                                .clipShape(Circle())
                        }
                    }

                    Text ("Your score: \(score)")
                        .font(.headline)
                }
                .foregroundColor(.black.opacity(0.6))
            }

            // game ended and show score
            if gameOver {
                VStack(spacing: 20){
                    Text ("Game Over!")
                        .font(.headline.weight(.bold))
                        .foregroundColor(.red)

                    Text (yourEvaluation())
                        .multilineTextAlignment(.center)

                    Button ("New game") {
                        withAnimation {
                            mainMenu()
                        }
                    }
                    .padding(10)
                    .background(.blue.opacity(0.4))
                    .foregroundColor(.yellow.opacity(0.8))
                    .clipShape(Capsule())
                    .transition(.scale)

                }
                .frame(width: 200, height: 200, alignment: .center)
                .background(.white.opacity(0.2))
                .clipShape(RoundedRectangle(cornerRadius: 30))
                .transition(.scale)
            }
        }
    }

    // set different views

    func mainMenu () {
        showMenuButtons = true
        showParameters = false
        gameHasStarted = false
        gameOver = false
    }

    func startGame() {
        round = 0
        score = 0

        generateQuestions()
        generateAnswers()

        gameHasStarted = true

        gameOver = false
        showMenuButtons = false
        showParameters = false
    }

    func setParameters(){
        showMenuButtons = false
        showParameters = true
        gameHasStarted = false
    }

    // actions

    func buttonTapped (_ button: Int) {
        let rightAnswer = questions[round].num1 * questions[round].num2
        if button == rightAnswer {
            score += 1
        }
    }

    func nextRound() {
        round += 1

        guard (round < (numberOfQuestions)) else {
            gameOver = true
            return
        }

        generateAnswers()
    }

    // initialize game play

    func generateQuestions () {
        var questionsGenerated = [(num1: Int, num2: Int)]() // tuple of random multipliers
        for _ in 1...numberOfQuestions {
            questionsGenerated.append((
                num1: Int.random(in: 2...difficulty),
                num2: Int.random(in: 2...difficulty)))
        }
        questions = questionsGenerated
    }

    func generateAnswers () {
        let myQuestion = questions[round]
        let rightAnswer = myQuestion.num1 * myQuestion.num2

        // make some unique answers
        var myAnswers = Set<Int>()
        myAnswers.insert(rightAnswer)
        repeat {
            myAnswers.insert (rightAnswer + Int.random(in: -20...20))
            myAnswers = myAnswers.filter{$0 > 3}  // cut off results < 4
        } while myAnswers.count < 5

        // save answers in mixed form
        answers = Array(myAnswers).shuffled()
    }

    // calculate result message

    func yourEvaluation() -> String {
        var output: String = "You scored \(score) of \(numberOfQuestions)!\n"
        if score <= 1 {
            output += "Serious?"
        } else if score <= (numberOfQuestions / 2) {
            output += "Well..."
        } else if score <= (numberOfQuestions / 4 * 3) {
            output += "Not that bad!"
        } else if score < numberOfQuestions {
            output += "Great!"
        } else {
            output += "Perfect!"
        }

        return output
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}