An airplane flew for 8 hours at an air speed of x miles per

An airplane flew for 8 hours at an air speed of x miles per hour (mph), and for 7 more hours at 325 mph. If the average airspeed for the entire flight was 350 mph, which of the following equations could be used to find x ?
A. x + 325 = 2(350)
B. x + 7(325) = 15(350)
C. 8x

Solution

let r = rate
let h = time in hours
let d = distance

total time it took was 8 + 7 = 15 hours.

average speed for the whole flight was 350 miles per hour.

formula for the entire flight becomes:

350 * 15 = d

formula for the first leg of the trip is equal to:

x * 8 = d1 where d1 is the distance flown for the first leg of the trip.

formula for the second leg of the trip is equal to :

325 * 7 = d2 where d2 is the distance flown for the second leg of the trip.

since we know that d1 + d2 = d, then we have a general formula that states

x * 8 + 325 * 7 = 350 * 15

that looks very much like selection E which is:

8x + 7(325) = 15(350)

your answer is selection E.