Line Plots · Hard problem

A bar graph shows how many cars of each color passed the school. Its grid lines count by $4$s: $0, 4, 8, 12, 16, 20$. The red bar ends on the $20$ line. The blue bar ends exactly halfway between the $12$ line and the $16$ line. The green bar is not drawn, but green had $3$ more cars than blue. How many red, blue and green cars were there in all?