There are a few ways that they can give you the information in one of these multiple-groups problems:
Group 1 TOTAL
Group 1 ONLY
This one gives us group X "ONLY", so we have to adjust for that in the formula. The people who used ONLY X represent the "X total" group minus the "both" group.
Group X then represents the group who ONLY used X (25%) and the group that used both X and Y (25%), so of the 160 people who used any product, 50% used X.
So group x is 80 people:
Group X + Group Y - Both + Neither = 200
80 + Group Y - 40 + 40 = 200
80 + Y = 200
Y = 120 (answer choice D)
Just as in the Venn Diagram problem in your previous post, they're not testing your ability to plug numbers into the formula as much as they're testing your ability to determine what the numbers represent. This is much more a test of how you think than of what you know, so knowing the formulas is only about 10% of the battle...the rest comes down to slowing down, determining what each value represents, and thinking critically about how to arrange the information to determine the answer.