Mathnasium #MathTricks: Number Sense (Multiplication Part 2)

By Mathnasium | Added Mar 2, 2022


Welcome to Mathnasium’s Math Tricks series. When kids learn their multiplication facts, they often first master their five times tables. But multiplying large numbers by five can get tricky. Today we are using the fact that five is half of ten to multiply an even or odd number by five.

To calculate any number times five, it can be easier to multiply by ten first and then divide the product by two. Multiplying a whole number by ten follows a simple visual pattern. To divide by two, we rely on our ability to find half of any number. Another option for calculating a number times five is to cut it in half *first*, then multiply by ten.

The general rule for our trick: when we multiply an even number by five, we find half first, then multiply by ten, and when we multiply an odd number by five, we multiply by ten first, then find half. Follow the examples below as we multiply an even and an odd number by five.

Example 1: Multiply 72 x 5.

   Step 1: Identify the number you are multiplying by five.

72 (even number)

   Step 2: Cut the number in half.

Half of 72 = 36

   Step 3: Multiply the new number by ten for the final answer.

36 x 10 = 360

   Answer: 72 x 5 = 360

Example 2: Multiply 95 x 5.

   Step 1: Identify the number you are multiplying by five.

95 (odd number)

   Step 2: Multiply the number by ten.

95 x 10 = 950

   Step 3: Cut the new number in half for the final answer.

Half of 950 = 475

   Answer: 95 x 5 = 475

Now, with this strategy, you are ready to multiply any even or odd number by five. Click here for more practice problems.

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