SOURCE

var arr = []
for (var i = 1; i <= 30; i++) {
    arr.push(2 * i)
}
console.log(arr)

var newArr = []
var add = 0

for (var j = 0; j < arr.length; j++) {
    add += arr[j];
    if ((j + 1) % 5 === 0) {
        newArr.push(add / 5)
        add = 0
    }
}
console.log(newArr)