Is this for MC II-A? Please try to put the course title in your subject/question as well.

Just to make it clear, we're comparing the two rules:

• $10a+b$ is div. by 7 if and only if $a-2b$ is div. by 7
• $10a+b$ is div. by 7 if and only if $3a+b$ is div. by 7

Do you think one is easier to use than the other? Why?