If 75 percent of the guests at a certain banquet ordered dessert, what percent of the guests ordered coffee?

1.) 60% of the guests who ordered dessert also ordered coffee.
2.) 90% of the guests who ordered coffee also ordered dessert.

Let D = total who ordered dessert.
Let C = total who ordered coffee.
Let B = total who ordered both dessert and coffee.
Plug in guests = 100.

Then D = .75*100 = 75.

Statement 1: 60% of the guests who ordered dessert also ordered coffee.
B = .6*75 = 45.
Not way to determine C.
Insufficient.

Statement 2: 90% of the guests who ordered coffee also ordered dessert.
.9C = B.
Not way to determine C.
Insufficient.

Statements 1 and 2 together:
.9C = 45.
C = 50.
Sufficient.

The correct answer is C.

The solution tells us the following:
45 ordered both coffee and dessert.
50-45 = 5 ordered only coffee.
75-45 = 30 ordered only dessert.
Thus, 100-45-5-30 = 20 ordered neither coffee nor dessert.
The solution does not assume that every guest ordered coffee or dessert. Given the values above, 20 ordered neither.