The raw sugar content of sugar water is 25/( 100+25)=20%. After drinking half of it, the sugar remains 25 * (1-1/2) =12.5 (g) water remains 100 * (65438).

In order to make the sugar water in the cup as sweet as before, the sugar content should still be 20%, and water should account for 1-20%=4/5.

86/(4/5)= 107.5 (g) (sugar water) 107.5*20%=2 1.5 (g) (sugar) plus 21.5-/kloc-

The reason for the wrong answer is that he directly takes 36*20%=7.2 (grams).