How to Add Multiples on the GMAT
Consider the following question from the Economist GMAT Tutor:
Zeta took several pictures with her new digital camera. Each picture was saved as a file, the size of which depended on the picture’s resolution. A low-resolution image requires a 0.5-megabyte file, and a high-resolution image requires a 0.9-megabyte file. If the total size of all the files was 8.1 megabytes, which of the following is a possible number of pictures Zeta took with her camera?
How to add multiples quickly
- Rewrite the question in the form of an equation:
- Multiply by to remove the decimal points
With the equation written like this, we are solving for + , or the total number of pictures.
Apply the appropriate rule: A multiple of multiple of multiple of .
is a multiple of . is also a multiple of . Therefore, must also be a multiple of .
Since must also be a multiple of , must be a multiple of . The only possible value for can be , since or any larger multiple is larger than any of the answer choices.
So, if , let’s solve for :
With and , then , which is answer choice C.
In summary, applying the rule that when you add two multiples of , the result will be a multiple of helped us to solve this question quickly. It’s a good rule to remember!
This post appeared first on the Economist GMAT Tutor blog.