How many more balloons must David give Robert ?

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?

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Ans: 20
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.