Tia was flying her model airplane. it took the airplane 5 seconds to go 90 feet. what was the average rate of speed of the model airplane, in feet per second?

By definition, the speed is given by the quotient between the distance traveled and the time it takes to travel that distance. We have then that the expression for speed is as follows
 v = d / t
 where
 d: distance
 t: time
 Substituting
 v = (90feet) / (5s) = 18feets / s
 answer
 the average rate of the airplane model, in feet per second was 18feets / s

Ithink your answer is b