How do you solve this question?
A cookie jar contains only two types of cookies - macaroons and ginger snaps. The ratio of macaroons to ginger snaps is $2:5$. Which of the following could be the total number of cookies in the cookie jar?
[X] 42
[ ] 45
[ ] 48
[ ] 52
[ ] 57
Why is the answer $42$?
Because it is the only multiple of $7=2+5$: $2$ parts of macaroons and $5$ parts of gingers.
The total number of biscuits must be $2n+5n=7n$, with $n$ integer, and $42$ is the only multiple of $7$ in your list.