let's call airplanes "a" and helicopters "h".
The total number of models is 35. This means that a+h=35
4 times as many model airplanes as model helicopters. This means a=4h.
we can substitute this into the previous equation:
4h+h=35
5h=35
dividing by 5:
h=7
so he has 7 helicopters and 4*7=28 airplanes
