Many numbers have special properties that make multiplication and/or division very easy, if we first recognize their properties. For this lesson, we will capitalize on the fact that 50 = 100 ÷ 2, and that 25 = 100 ÷ 4. It is pretty easy to divide a number by 2 or 4, and it is even easier to multiply a number by 100 (just add 2 zeros at the end!). Simply combine these two steps, and you can multiply by 50 or 25. Together, Mult50 and Mult25 showed up 11 times on middle school tests this year.
Number Dojo Levels: 7, 9
To multiply a number by 50:
 Divide it by 2.
 Multiply by 100. You're done.
Example 1: 50 x 48
 Divide by 2. 48 ÷ 2 = 24.
 Multiply by 100. 24 x 100 = 2400.
Example 2: 72 x 50
 72 ÷ 2 = 36.
 36 x 100 = 3600.
Example 3: 50 x 27
 27 ÷ 2 = 13.5
 13.5 x 100 = 1350. Notice that the original number was odd, so we came up with a decimal quotient instead of a whole number. In these cases, we move the decimal to the right two places (instead of simply adding 2 zeros at the end).
To multiply a number by 25:
 Divide it by 4 (or by 2 twice).
 Multiply by 100. Yes, it's that easy.
Example 4: 28 x 25
 Divide by 4. 28 ÷ 4 = 7.
 Multiply by 100. 7 x 100 = 700.
Example 5: 25 x 116
 116 ÷ 4 = 29. (It might be easier for you to divide 116 by 2 twice. 116 ÷ 2 = 58. 58 ÷ 2 = 29).
 29 x 100 = 2900.
Example 6: 53 x 25
 53 ÷ 4 = 13.25 (or 53 ÷ 2 = 26.5. 26.5 ÷ 2 = 13.25).
 13.25 x 100 = 1325 (simply move the decimal two places to the right.
Example 7: 25 x 219
 219 ÷ 4 = 54.75 (or 219 ÷ 2 = 109.5. 109.5 ÷ 2 = 54.75).
 54.75 x 100 = 5475.
These concepts take a little practice. Please download my worksheets below!

