Remember that Distance = Speed x Time
When the 2 Balloons meet, they will be at the same height FROM THE GROUND :
Distance travelled by #1 : 10 + 15.Time
Distance travelled by #2 : 150 - 20.Time (the descending speed is negative)
Since it's the same distance from the ground, we can write:
10 + 15.Time =150 -20.Time
15.Time + 20.Time = 150 - 10
35.Time = 140
And Time = 4 minutes
Now take any equation and plug in Time = 4:
10 + 15.Time = 10+15x4 = 70 meters
OR
150 - 20.Time = 150-20x4 = 70 meters
