Our base-$10$ system makes it easy to check whether a number is divisible by certain numbers. For example, checking whether $356$
is divisible by $2$ or $5$ comes down to checking whether the *last* digit is a multiple of either number. In this case, our number is divisible by $2,$ but not by $5.$

*Why* does this work, though? Are there similar shortcuts for $4$ and other numbers? Keep reading to find out, or skip ahead to the challenge for a puzzle on this idea.