Question 167491
lets call # of cars and motorcycles, c and m respectively.

c+m=28  eq 1
c=3m    eq 2

take the value from eq 2 and place it in eq 1. 

3m+m=28---->4m=28--->{{{highlight(m=7)}}}motorcycles
:
{{{highlight(c=3(7)=21)}}}cars