6. A $33 \mathrm{cl}$ bottle of orange drink is composed of $75 \%$ water and $25 \%$ orange juice. Enrica wants to replace some of the drink in this bottle with orange juice, so as to obtain a new drink that is $50 \%$ orange juice. How many cl of the initial drink must Enrica replace with orange juice?
(A) 8.25
(B) 10
(C) 9.75
(D) 12
(E) 11
(6) The correct answer is $(E)$.

The bottle initially contains $99 / 4 \mathrm{ml}$ of water and $33 / 4 \mathrm{ml}$ of orange juice. After replacing $x \mathrm{ml}$ of the drink with orange juice, the drink will contain $99 / 4 - 3 x / 4 \mathrm{ml}$ of water. For the water to be half of the bottle's content, it must be $99 / 4 - 3 x / 4 = 33 / 2$, from which $x=11$.

Problem proposed by Carmelo Di Stefano.