13. A one-liter bottle of orange drink is made up of $80\%$ water and $20\%$ orange juice. Gianni wants to replace some of the drink in this bottle with orange juice, so that the new drink is $50\%$ orange juice. How many ml of the initial drink should Gianni replace with orange juice?
(A) 300
(B) 400
(C) 375
(D) 320
(E) 350
(13) The correct answer is $(C)$.

The bottle initially contains $800 \mathrm{ml}$ of water and $200 \mathrm{ml}$ of orange juice. After replacing $x \mathrm{ml}$ of the drink with orange juice, the drink will contain $800-4 x / 5 \mathrm{ml}$ of water. For the water to be half of the bottle's content, it must be $800-4 x / 5=500$, from which $x=1500 / 4=375$. Problem proposed by Carmelo Di Stefano.