One plane at 520 feet is ascending at a rate of 40 feet per minute. While another plane at 3800 feet is descending at a rate of 120 feet per minute. How long will take the two planes to be at the same altitude?

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