GMAT Club Official Solution:A festival organizer is putting together the final set of food trucks for a weekend street-food event. Several food trucks applied, and exactly 3 of them will be chosen. How many different groups of 3 food trucks could be chosen?(1) If 2 more food trucks had applied, the number of possible 3-truck groups would have been 120.
Let T be the total number of food trucks that actually applied. If 2 more food trucks had applied, the total would have been T + 2. So the number of possible 3-truck groups would have been:
(T + 2)C3 = 120
(T + 2)!/(3!(T - 1)!) = 120
(T + 2)(T + 1)T/6 = 120
There is no need to waste time trying to solve this equation algebraically. Notice that, as T increases, this expression increases. So there can be only one value of T that satisfies this equation. Therefore, statement (1) determines exactly one actual number of applicants, and hence exactly one value for the number of 3-truck groups in the original question.
Sufficient.
(2) If 2 fewer food trucks had applied, the number of possible 3-truck groups would have been 20.
If 2 fewer food trucks had applied, the total would have been T - 2. So the number of possible 3-truck groups would have been:
(T - 2)C3 = 20
(T - 2)! / [3!(T - 5)!] = 20
(T - 2)(T - 3)(T - 4) / 6 = 20
Again, as T increases, this expression increases. So there can be only one value of T that satisfies this equation. Therefore, statement (2) also determines exactly one actual number of applicants, and hence exactly one value for the number of 3-truck groups in the original question.
Sufficient.
Answer: D.