I used trial and error for this one.
tricycles= 3 wheels
bicycles= 2 wheels
t+b=15
tricycles wheels + bicycle wheels = 36
6 tricycles have 3 wheels each = 18
9 bicycles have 2 wheels each = 18
18+18= 36 wheels and 6+9=15 bicycles and tricycles
THEREFORE there are 9 bicycles
