Walking with 4/5 of my usual speed, I arrive at the bus stop 10 minutes later than normal. How many minutes does it take to walk to the bus stop at my usual speed?  Select the correct option.
A) 32 B) 36 C) 40 D) 44 E) 48

Rationale: "let t = usual time = distance / usual speed t + 10 = distance / ( 4 * usual speed / 5 ) = ( 5 * distance ) / ( 4 * usual speed ) = 5 t / 4 t = 40 the answer is c ."
Answer: C