Im receiving a error, and dont know how to discover what is wrong

fun main() {
    var a : Double = readLine()!!.toDouble().times(2)
    var b : Double = readLine()!!.toDouble().times(3)
    var c : Double = readLine()!!.toDouble().times(5)
    var str : String = String.format((a.plus(b.plus(c))).div(10.0).toString())

    println("MEDIA = $str")
}