During a sale, the cost of each shirt is $ 15 and that of each pair of pants is $ 25. Geoff can spend at most $ 120 at the store. If Geoff buys s shirts and p pairs of pants, which of the following must be true?

The correct answer is: 15s + 25p < = 120

Hint:-
Total Amount to be spent = Amount spent on shirts + Amount spent on pats < = $ 120
Amount spent on a particular item = Number of items * cost of each such item.
Step-by-step solution:-
Amount spent on shirts = Number of items × cost of each such item
                                     = s × 15
                                     = 15s ------------- (Equation i)
Amount spent on pants = Number of items × cost of each such item
                                      = p × 25
                                       = 25p ------------- (Equation ii)

Now, we know that Maximum Amount that can be spent by Geoff altogether is at most $ 120 i.e. $ 120 or less.
∴ Total Amount spent < = 120
∴ Amount spent on shirts + Amount spent on pats < = 120
∴ 15s + 25p < = 120 --------------- (From Equations i & ii)
Final Answer:-
    Option A i.e. 15s + 25p < = 120 is the correct Option.
Note:
Alternatively, we can use the elimination method in exam to save time by considering the following-
As per the conditions given in the question, Amount that could be spent is at most $120 i.e. $120 or less and the only 2 options that satisfy this condition are Options A & C.
Hence, Options B & D are eliminated.
Then, in Option A & C, while the RHS remains same i.e. Total Cost = $ 120, the coefficients of s & p as per the conditions given in the question should correspond with 15 and 25 respectively because of their per piece cost.
In option C it is interchanged (25s + 15p). Hence, Option C is also eliminated.
Hence, Option A is the correct option.