If each sugar cube has a mass of 3g, how many grams of sugar are there per 100mL of water in each cup?
The question is missing a crucial detail: concentration of sugar. This can be provided in terms of number of sugar cubes per cup (of a certain volume) or number of sugar cubes per 100 ml of water, etc.
For example, if we mix 3 cubes of sugar in a cup that has a volume of 250 ml, then we have added 3 cubes x 3 gm per cube = 9 gm of sugar in 250 ml of water. This means we have added 9 gm/250 ml x 100 ml = 3.6 gm of sugar per 100 ml of water.
Similarly, if the concentration was given in terms of say 2 cubes per 100 ml of water, it would have meant 2 cubes x 3 gm per cube = 6 gm sugar per 100 ml water in cup.
Thus, given a concentration of sugar per cup or per unit water, we can calculate the amount of sugar per 100 ml of water.
Hope this helps.