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prealgebra intermediate

Problem

In Perfectville, the streets are all feet wide and the blocks they enclose are all squares of side length feet, as shown. Sarah runs around the block on the -foot side of the street, while Sam runs on the opposite side of the street. How many more feet than Sarah does Sam run for every lap around the block?

problem
Solution
At each corner of the street, Sam runs 40 more feet than Sarah. Otherwise, Sam runs the same distance as Sarah. Since there are four corners, Sam runs more feet than Sarah does.
Final answer
160