Hoozan
IanStewart When we get 7a + 5b = 63 is there any way we could estimate to narrow down our possibilities?
For these types of questions, when the prices are very close together, estimation is usually the best strategy. Here by estimating, we only easily eliminate two answer choices -- if the person bought only apples, they would buy 9 apples, and if they bought only bananas, they would buy 12.6, so the answer must be between 9 and 12.6, and D and E must be wrong. If you combine that estimate with a bit of number theory, you can get the answer easily though:
7a + 5b = 63
2a + 5a + 5b = 63
2a + 5(a + b) = 63
Notice now that 2a is even, and 63 is odd, so 5(a + b) must be odd, so a+b is odd. And a + b is one of the answer choices, and the only odd answer we haven't eliminated yet (after estimating) is 11.
I still prefer the divisibility approach I explained on the previous page; with practice, that kind of approach can be used to solve this question in 15-30 seconds.