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?

Let the number of minutes when they will be at the same altitude =M

520 + 40M =3800 - 120M, solve for M

M =41/2 =20.5 minutes - when both planes will be at 1340 feet.