Mental math shortcut is one of the topics that we will be discussing in this blog. After discussing **multiplication by 11** and **squaring numbers ending in 5**, let us discuss about the shortcut of multiplying by 5.

The Mental Math Calculation

**Example 1:** How do you multiply 28 × 5?

Instead of multiplying 28 by 5,

(1) Multiply 28 by 10: **28 × 10 = 280 **

(2) Divide the product in (1) by 2: **280 ÷ 2 = 140**

**Example 2: **65 × 5

(1) Multiply 28 by 10: **65× 10 = 650**

(2) Divide the product in (1) by 2: **650 ÷ 2 = 325**

Notice that in dividing 650 by 2, you can split 650 into 600 and 50 and get half of each. Half of 600 is 300 and half of 50 is 25, so the quotient is 325.

**Example:**** 360 × 5**

(1) Multiply 360 by 10: **360× 10 = 360****0**

(2) Divide the product in (1) by 2: **3600 ÷ 2 = 1800**

The method above will work for larger number of digits. It is easy to master it so you can experiment with different numbers and explore how the shortcut works.

**The Secret Behind the Math Shortcut**

The secret behind the mental math shortcut is easy that even elementary school students can detect them. Multiplying a number by 5 is the same as multiplying by 10 and dividing by 2. If we let n be the number, then multiplying by 10 (Step 1) gives a product of 10*n.* Now, if we divide 10*n* by 2 (Step 2), the result is 5*n* which is n multiplied by 5.