Can someone with this math question

DC-10
At t=0 the altidude is 4500 feet
It's descending at a rate of 150 feet per minute, then each 5 minutes it descends (150 feet/min)(5 min)=750 feet. Then:
At t=5 the altitude is 4500 feet-750 feet=3750 feet
At t=10 the altitude is 3750 feet-750 feet=3000 feet

727
At t=0 the altidude is 600 feet
It's climbing at a rate of 75 feet per minute, then each 5 minutes it climbs (75 feet/min)(5 min)=375 feet. Then:
At t=5 the altitude is 600 feet+375 feet=975 feet
At t=10 the altitude is 975 feet+375 feet=1350 feet

The graph with these characteristics is Graph B.

The graph B shows when the two planes will be at the same altitude 
 

What mathematical statement is missing ? ? the answer is b