Walking with 4/5 of my usual speed, I arrive at the bus stop 9 minutes later than normal. How many minutes does it take to walk to the bus stop at my usual speed?  Choose the correct choice from the following.
A) 24 B) 28 C) 32 D) 36 E) 40

Rationale: "let t = usual time = distance / usual speed t + 9 = distance / ( 4 * usual speed / 5 ) = ( 5 * distance ) / ( 4 * usual speed ) = 5 t / 4 t = 36 the answer is d ."
Answer: D