Find first number that is smaller than some number but divisible with other number

Originally Posted by Glenn Kennedy

I just overcomplicated it... as usual... and got it incorrect at the same time!!

However, do check out Fluff13's formula with 200 (or any multiple of 20) as the input number. It fails and falls over. Use FLOOR or Ali's suggestion.

I checked Fluff13 formula with 200 and 20 and result is 180, which is ok. Am I missing something?