Let

v = the speed of the propeller plane

2v = the speed of the jet plane ( times faster than the propeller plane)

d = 1500 mi the distance traveled 

t = the time of the jet

t + 3 = the time of the propeller plane (3 hour longer than the jet)

A jet flies twice a a fast as a propeller plane means:

Since distance = speed*time we have:

v(t + 3) = d

2vt = d follows t = d/(2v)

Plug t = d/(2v) into v(t + 3) = d

v(d/(2v) + 3) = d

Plug d = 1500

v(1500/(2v) + 3) = 1500

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click here to see the equation solved for v

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v = 250 mph the speed of the propeller plane

2v = 2*250 = 500 mph the speed of the jet plane

The speed of the propeller plane was 250 mph.

The speed of the jet plane was 500 mph.