Shockwave Question: Solving Mach 3 Plane 20Km Above Person

[AriAstronomer]

Homework Statement

A supersonic jet traveling at Mach 3.00 at an altitude of
20 000 m is directly over a person at time t ϭ 0, as in
Figure P17.43. (a) How long will it be before the person
encounters the shock wave? (b) Where will the plane be
when it is finally heard? (Assume that the speed of
sound in air is 335 m/s.)

Homework Equations

The Attempt at a Solution

So I figured it would just be the time for sound to travel from the plane to the ground, or t = d/v = 20 000m/355m/s, but that gives me 59.7s, and the answer book says 56.3s. What am I doing wrong?

 

[rickz02]

Check the given speed of sound and recalculate...