Riddle: Robert and David were preparing to have a water balloon fight.
"No Fair" cried Robert, "You have 3 times as many as I do!"
David said "Fine!" and gave Robert 10 more balloons.
"Still not fair!" argued Robert, "You still have twice as many as I do."
How many more balloons must David give Robert for them to have the same number?
Answer: David must give Robert another 20 water balloons, giving them each 60. Robert started with 30 water balloons and David with 90.