Wow that's a great mixture

Intially the mixture is $\frac{9}{13}$ grape, then, adding the water it is half that, or $\frac{9}{26}$ grape. Now $40\%$ of this mixture is consumed leaving $\frac{3}{5}$ of the $\frac{9}{26}$ or $\frac{27}{130}$ of a full glass being grape.

Next $40\%$ of a full glass in a ratio $\frac{6}{13}$ or $\frac{2}{5}\frac{6}{13}=\frac{24}{130}$ of grape out of a full glass is added to the original, yielding $\frac{27}{130}+\frac{24}{130}=\frac{51}{130}=39.23\%$

Its too simple, treat 1 part as 100 ml. Now first I took grape to wine in 9:4=900ml:400ml , now this is half of the glass, which means other half(1300 ml) is filled with water. Now 40% of this mixture I drank, left over is in the ratio, (540:240:780). Now 40%= 1040 ml, and we have to add again a mixture of grape wine and orange wine in ratio 6:7 respectively with respect to 1040 ml, which is (480:560). Now adding it to the left over mixture we get a ratio as 1020:800:780 . Now easily you can calculate the grape wine percent. (1020/ 2600).