Thought of posting my solution to the challenge in the WeSplit: Wrap up, including the currency formatter closure.

I've left a couple of lines commented out -- those were to have the segmented picker for the tip percentage.

struct ContentView: View {
    @State private var checkAmount = 0.0
    @State private var numberOfPeople = 0
    @State private var tipPercentage = 20
    @FocusState private var amountIsFocused: Bool

//    private let tipPercentages = [10, 15, 20, 25, 0]
    private let tipPercentages = 0...100

    private var totalPerPerson: Double {
        let peopleCount = Double(numberOfPeople + 2)
        let amountPerPerson = grandTotal / peopleCount

        return amountPerPerson
    }

    private var grandTotal: Double {
        let tipPercentage = Double(tipPercentage)

        let tipValue = tipPercentage * checkAmount / 100
        let grandTotal = checkAmount + tipValue

        return grandTotal
    }

    private var currency: FloatingPointFormatStyle<Double>.Currency {
        .currency(code: Locale.current.currency?.identifier ?? "USD")
    }

    var body: some View {
        NavigationView {
            Form {
                Section {
                    TextField("Amount", value: $checkAmount, format: currency)
                        .keyboardType(.decimalPad)
                        .focused($amountIsFocused)
                }

                Picker("Number of people", selection: $numberOfPeople) {
                    ForEach(2 ..< 100) {
                        Text("\($0) people")
                    }
                }

                Section {
                    Picker("Tip percentage", selection: $tipPercentage) {
                        ForEach(tipPercentages, id: \.self) {
                            Text($0, format: .percent)
                        }
                    }
//                    .pickerStyle(.segmented)
                } header: {
                    Text("How much tip do you want to leave?")
                }

                Section {
                    Text(grandTotal, format: currency)
                } header: {
                    Text("Grand Total")
                }

                Section {
                    Text(totalPerPerson, format: .currency(code: Locale.current.currency?.identifier ?? "USD"))
                } header: {
                    Text("Amount per person")
                }
            }
            .navigationTitle("WeSplit")
            .toolbar {
                ToolbarItemGroup(placement: .keyboard) {
                    Spacer()

                    Button("Done") {
                        amountIsFocused = false
                    }
                }
            }

        }
    }
}