Please repost with the complete statement of the problem.
Need to know how long was the return trip against the wind.
R is the speed of the plane.
S is the speed of the wind.
T is the TOTAL time in hours
There are two equations:
Distance = rate x time
= (R + S) x 1.5 <--- with the wind
= (R - S) x (T- 1.5) <--- against the wind
Note that 90 min - 1.5 hours