Answer :
We suppose that Tim has x postcards. So Shawn has 3/5x postcards and Paul has 2/3*3/5x=2/5x postcards. Then the total is x+3/5x+2/5x=2x=280. So the x=140 which means Tim has 140 postcards and Paul has 140*2/5=56 postcards. So the answer is 140-56=84.