multiply by something
to use the arithmetic process of multiplication to expand numerically a certain number of times. To get the amount of your taxes, multiply by .025. Can you multiply by sixteens?
multiply something by something
to use the arithmetic process of multiplication to expand numerically a particular number a certain number of times. Multiply the number of dependents you are claiming by one thousand dollars. Multiply 12 by 16 and tell me what you get.