I found this question in Greg’s video on Arithmetic fundamentals series (towards the end of the 3rd session) but I just CANNOT seem to understand how he solved it. Is there a simpler way to solve this?

can you show your rough work ?

I actually just brute forced it. I know that between 1-10 theres no such integers and then I basically checked for all odd (non-prime) integers. This doesn’t seem right.

Brute force is also good for this question since the range is given and its small !

But here is the other way to think about it:

https://gmatclub.com/forum/how-many-integers-less-than-100-have-exactly-4-odd-factors-b-97404.html