Hi LulaBrazilia,
Each of the explanations in this string focused on "prime factorization", which is a MUST to solving this question.
The reason you KNOW that prime factorization is necessary is in the wording and "design" of the question. Here's why:
The question asks for THE sum of the biggest and smallest variables, NOT what COULD BE the sum. Since 1<w<x<y<z there must be a fixed value for each of the variables, meaning that they're most likely prime numbers. If they weren't prime, then they'd have factors and it's possible that the variables (especially the z) could change.
For example, the numbers 2, 6, 8 and 10 would have a product of 960. So do the numbers 2, 3, 8 and 20. In this scenario z could be different values, which would change the answer to the question. But this scenario does not match the wording of the prompt.
When thinking about how to solve a problem, pay attention to the specific wording of the prompt. GMAT questions are very carefully worded and often provide clues as to how to approach the problem (and sometimes the easiest approach to it).
GMAT assassins aren't born, they're made,
Rich
