Find two numbers whose product is 42 and difference is 1.

Okay, So We Need Two Numbers That Multiply To Get 42. Lets Look At All Of The Ways To Get 42.
1 * 42 
2 * 21
6 * 7
7 * 6 
21 * 2
42 * 1

So, 21 and 2 don't have a difference of one. 
42 and 1 don't have a difference of one. 

6 and 7 have a difference of one, which means your two mystery numbers are 6 and 7.

Maybe the answer is 20mm. all i really did was just multiply, which is obvious.