Hot air balloon
700 because the balloon has to climb up to 800.
It takes 39 minutes for the balloon to reach the attitude of 20. It takes 80 minutes to land because its 40 minutes to get to the top and 40 more minutes to get down.
y=1,200-20x. It takes 60 minutes land. The balloons descend at the same rate.
y=800-30x. It takes 26.665 minutes to land. Both of the balloons lower at the same rate.
y=30x. The two balloons would meet at 16 feet. The altitude the two would meet at would be 480 feet.
The thrird balloon would have to begin its decent at 13.34.