1. To use multiplication according to a particular factor (specified after the word "by"). Finally, we have to multiply by seven to find the solution. If you multiply by 0.33, you'll get the portion of your rent that you are allowed to write off as a business expense.
2. To use multiplication on a given number according to a particular factor (specified after the word "by"). In this usage, a noun or pronoun is used between "multiply" and "by." Multiply that number by 10—that's how much you owe me now! If you multiply nine by any single digit, the product will always be two digits that, when added together, equal nine!
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.
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?