A hot air balloon is 10 meters above the ground and rising at a rate of 15 meters pre min. Another ballon is 150 meters above the ground and descending at a rate of 20 meters pre min . When will the two balloons meet ?

Answer :

I think they will meet in 4 minutes.

Explanation
10+15+15+15+15=70
150-20-20-20-20=70
Every minute the first balloon goes up 15 meters and the second balloon goes down 20 meters.