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?
They had 90 and 30 at first.
David gave 10 to Robert then they had 80 and 40.
So David must give 20 more and they'll have 60 each.