Its a very straight forward question:

All you would do is the following:
Cost of Standard Shirt (S.S) = $100
Cost of Fitted Shirt (F.T) (100*1.5)= $150

Now you have F.T sold:
3,4,5,6,7,9,12,15 & 17

Total Revenue on a given day = $2450
So we know unit cost of F.T is $150. So all you need to do is multiply the above numbers the following way

Assuming 3 F.T were sold = 3*150 = $450
Now S.S sold dollar worth = 2450-450 = $2000
Check if S.S sold is divisible by unit cost of S.S. In this case 2000/100 = 20 S.S shirts sold so 3 is CORRECT.

Do the same for the remaining numbers and answer is 3,5,7,9 & 15.

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