This was fun!
First, looking at the ones column. M - M has to be zero. so L is 0.
Rewrite:
K0M
-MM
--------
K0
Ok, now you have a zero in the tens column in the top line so you know you have to regroup from the 100's column to subtract M from 0. But you do not get anything in the 100's column in the answer! So you have used it all up when you regroup.... therefore you only had a 1 in the 100's column. So K = 1. Rewrite:
10M
-MM
-------
10
M must be 9 to get 10-9=1 in the tens column. So the final problem rewritten is:
109 - 99 = 10, which is (thankfully) true.
