MathAlready the calculator for everyone

Viswam walks half a mile to get to school each day. His route consists of 10 city blocks of equal length and he takes minute to walk each block. Today, after walking 5 blocks, Viswam discovers he has to make a detour, walking 3 blocks of equal length instead of 1 block to reach the next corner. From the time he starts his detour, at what speed, in mph, must he walk, in order to get to school at his usual time?