[vorg]

> 2040 is, which is divisible by 7 iff 204 is

I don't follow that one.

To check whether 2747 is divisible by 7, I divide the multiple of 100 (i.e. 2700) by 50 (i.e. 54), add it to the remaining two digits, 47 (i.e. 101), then see if that's divisible by 7.

[ohwaitnvm]

He reduced 2040 to 204 by dividing by 2 and 5 because he’s not interested in the factoring so much as just whether 7 is a factor.

[CurtMonash]

Correct.

If I'm checking for any prime other than 2 or 5 -- each of which has it own quick-check anyway via the last digit only -- it's always OK to drop trailing zeros.