zenefits oa - random(5) to generate a random(7)

If given a function that generates a random number from 1 to 5, how do you use this function to generate a random number from 1 to 7 with the same probability? (ie. function a has probability 1/5 per number, function b would have probability 1/7).  

int sum = 0;
int times = 5;
while(times>0){
　　times--;
　　sum += rand(5);
}

// 5 ~ 25  = 3 *７
return (sum - 2) / 3;

zenefits oa - random(5) to generate a random(7)

原文：http://www.cnblogs.com/marc-leetcode/p/5175013.html