10 dollars; because 10 is half of 20. When working out percentages, you have to think of it as in 100% = 1 whole of something. So 50% (Which is half of 100%) would be half of something. And so forth 25% (One quarter of 100%) would be one quarter of something. (Divide it by 4) Hope this helped with understanding percentages a little more.
(Can others please check this i have numerical dyslexia and may have made mistakes)