Juice Box

[baha777]

I got this from some other website, but didn't quite understand the way they explained the answer. Thanks

A man buys some juice boxes. The boxes are from two different brands. How many boxes of brand A did the man buy if he bought $5.29 worth of boxes?
(1) The price of brand A box is $0.81 and the price of brand B box is $0.31
(2) The total amount of boxes is 9


OA: A

[riteshbindal]

See, the total worth is 5.29.
A: One is .81 and second is .31. A sum of multiple of these numbers should give 5.29. It implies that the sum should be either 9/19/29... Reason being that both the prices are ending with 1 and to make a total of sum ending with 9, the total number of boxes should be 9 or 19 etc.
Anyways, the statement becomes:
.81x + .31 y = 5.29.
Now, if the total number is 19 (assumption). Min amount should be .31 * 19 which is roughly 5.7 and in turn greater than total price of 5.29, so the total number should definitely be 9.
and here is the equation.
x + y = 9
.81x + .31y = 5.29.
So A is sufficient.

B: Total number of boxes is 9. This is pretty obvious. It doesn't give any clue of A and B so this is insufficient.

Hence answer is A.