/Swift/Left side of mutating operator isn't mutable: 'self' is immutable

Left side of mutating operator isn't mutable: 'self' is immutable

   mutating func nextQuestion(){ // add "mutating" before func to change value of variables from (the struct)
        if questionNumber + 1 < questions.count { // to count answered questions
            questionNumber += 1
        }else{
            questionNumber = 0
        }
    }

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