Non-factors of factorials

GRE Quant Problem Solving
1

Hello there,

Could someone explain why the trick of finding the non-factors of factorials using primes only work for numbers greater than 10! as noted in the following screenshot of the Quant mountain.

image
image649×569 40.4 KB

I have tried 3! and I found that 4 is not a factor of it. But what I don’t get is what’s so special about 10? Thanks.

2

Because the method above is not rigorous and does have false-positives. I recommend playing with Composite Non-Factors Visualizer

3

interesting stuff