一道英文的JS题目,求助

Given the function foo() which takes an integer argument and returns an integer which is equal to the argument with unit and ten digit switch places. For example, foo(125) is 152, foo(5) is 50, foo(30) is 3 andfoo(1273)is 1237. What is the value of num at the end of the program segment?
num = 17;
i = 0;
while (i<4) {
num = foo(num) * 2;
num++;
i++
}
完全不明白............连题目都没看懂.............
求助,谢谢