What is the % mass of a solution composed of 265mg of glucose dissolved in 3 grams of water?

8.1%

Explanation:

mass of glucose / total mass of solution x 100

So first 265mg into grams = 0.265 g

0.265 + 3 = 3.265

mass of glucose = 0.265

total mass of solution = 3.265

0.265/3.265 x 100 = 8.1% (rounded to 2 s. f)