A commercial jet has been instructed to climb from its present altitude pf 9000 feet to a cruising altitude of 44000 feet, if the plane descends at a rate of 2,500ft/min how long will it take to reach its cruising altitude?

Firstly we need to count what is the difference between both altitudes:

44,000 - 9,000 = 35,000

If a plane descends at a rate of 2,500 feet per minute, then we have to count how many 2,500s fit in 35,000:

35,000 / 2,500 = 14

It means that the jet will reach its cruising altitude in 14 minutes.