Dividing by Dropping Digits
An integer decreases an integral number of times when its last D digits are
dropped. Obviously, there are infinite such integers, as any integer
ending in D 0's decreases by a factor of 10^D when the training 0's are
dropped. Is the number of such integers finite, if the last D digits are
no all 0's? If it is finite, how many such integers exist for D=1,2,3,4?
Source: Sudipta Das.
Mail to Ken