It takes 20.5 minutes for the two planes to be at the same altitude.
Step-by-step explanation:
Let 't' be time at which the two planes be at the same altitude.
- One plane at 520 feet is ascending at a rate of 40 feet per minute.
⇒ 520 + 40t
- Another plane at 3800 feet is descending at a rate of 120 feet per minute.
⇒ 3800 - 120t
When two plane be at the same altitude,
⇒ 520 + 40t = 3800 - 120t
⇒ 160t = 3280
⇒ t = 3280/160
t = 20.5 minsutes
