so 30 minutes=1/2 hour
7200 per hour
7200 times number of hours=height of first hot air ballon
4000 per hour
4000 times numbe rof hours=height of second ballon
1/2 hour
7200 times 1/2=3600
4000 tiemes 1/2=2000
they will never be since the first one will acent much faster that the other