The base case is easy, just take q,r=0.
Assume its true for all integers up to a fixed n. Then write n=qb+r for non negative integers q,r and 0 <= r < b.
Then n+1 = qb + (r + 1). Since between any two consecutive natural numbers there are no others, we get that r<b implies r+1 <= b. So either r+1=b or r+1 < b.
The rest should be cake.