Tom fills a 250 pound sandbag 80% full.  But the filling material he is using is 40% heavier than sand.  How much does the bag weigh?
The bag full of sand would weigh 250 * .8 = <T>multiply(250, 0.8)=200.0 pounds
The filling material would make it 200 * .4 = <T>multiply(200, 0.4)=80.0 pounds heavier
So it weighs 200 + 80 = <T>add(200, 80)=280 pounds
