In the previous post, we have learned how to **calculate the percentage of a number** by ignoring the percent sign. We continue our discussion on the tricks in getting the percentage with a number by making use of 10%.

Calculating 10% of the number is one of the easiest calculations in percentage. To calculate 10% means to get 1/10 of that number. This can be done easily mentally: 10% of 200 is 20 and 10% of 85 is 8.5. As we can see, we just eliminate 0 in the ones place if the number ends with 0 or move the decimal point one digit to the left. So how do we make use of 10% in making calculations?

Example 1: Multiplies of 10%

What is 20% o 750?

We know that 10% of 750 is 75 and 20% is twice 10%. Therefore, we just have to multiply 75 by 2 which is equal to 150. So, 20% of 750 is 150.

Example 2: Divisors of 10%

What is 15% of 120?

10% of 120 is 12. But what about 15%? Well, we know that 5% is half of 10% and 10% of 120 is 12, so 5% is 6. So, we just add 12 and 6 and we get 15% of 120. Therefore, 15% of 120 is 12 + 6 = 18.

Notice that this calculation can be done mentally with enough practice.

Example 3: Combining Multiples and Divisors of 10%.

What is 35% of 80?

We know that 10% of 80 is 8. So, 30% of 80 is 24.

Now, 5% is half of 10%, so it is half of 8 which is equal to 4.

So, 35% of 80 is 24 + 4 = 28.

As we can see from the above examples, we can use 10% of a number and then its multiples in order to calculate mentally. This can be done easily with constant practice.

**Why the Mental Math Trick Works**

There is really no secret in this trick but the simple concept of divisors and multiples. I leave it to you to figure them out. 🙂