Sarah walks 1 mile to get to the library each day. Her route consists of city blocks of equal length and she takes seconds to walk each block. Today, after walking blocks, Sarah discovers she has to make a detour, walking blocks of equal length instead of block to reach the next corner. From the time she starts her detour, at what speed, in mph, must she walk, in order to get to the library at her usual time? 
8
8.5
9
9.5
10
