...

foo.swift

protocol Addable {
    func +(_: Self, rhs: Self) -> Self
}

protocol Divisable {
    func /(_: Self, rhs: Self) -> Self
}

protocol InitializableByInt {
    init(_: Int)
}

extension Array where Element :
    protocol<Addable, Divisable, InitializableByInt> {
    func average() -> Element? {
        var sum : Element
        if self.isEmpty {
            return nil
        }
        else {
            sum = self.first!
            for i in 1 ..< self.count {
                sum = sum + self[i]
            }
            return sum/Element(self.count)
        }
    }
}

// or, in shorter form
extension Array where Element :
    protocol<Addable, Divisable, InitializableByInt> {
    func average() -> Element? {
        guard !self.isEmpty else { return nil }
        return self.reduce(Element(0), combine: +) / Element(self.count)
    }
}

// note, however, that you'd need to explicitly conform types to
// your custom protocols, which makes it much easier to simply make
// use of the Swift native protocols.
extension Double: Addable, Divisable, InitializableByInt { }
// ...

Still I am getting the same error as i mentioned you, what should I do?
after this
var array1 = [10,20,30]
array1.average() //error line