How can you get 4 from 5?
How can you give someone $63 using six bills, without using any one dollar bills?
1 - $50 bill, 1 - $5 bill , 4 - $2 bills
One out of 9 otherwise identical balls is overweight. How can it be identified after 2 weighings?
Weigh a group of 3 balls against another group of 3, then you'll know which group of 3 contains the heavy ball. Pick 2 balls from that group and weigh one against the other.