Or the way I do it:

• 3 * 20 = 60
• 60 - 51 = 9
• 20 - (9 / 3) = 17

So the factors are 17 and 3. I know 3 is a factor because 5+1=6, which is divisible by 3, so I just use a convenient multiple of 3 that’s pretty close to the actual number to get the divisor.

I have young kids and they keep asking me to do crazy math problems while driving, so that’s generally the trick I use.