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Spot the error

This Swift code contains one error – can you select the line that is incorrect?

Tip: You should mentally run the code from top to bottom, meaning that if line 3 says an integer should be returned and line 10 tries to return a string, line 10 is the wrong one.

function performExercise(count: Int, type: String) -> String {
    var output: String = ""
    for _ in 0 ..< count {
        output += "I'm doing a \(type)\n"
    }
    if count < 10 {
        output += "That was easy!"
    } else if count < 30 {
        output += "That was challenging!"
    } else {
        output += "Phew - I need a break!"
    }
    return output
}
print(performExercise(count: 50, type: "crunch"))
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