Percentage Math Tricks: Making Use of 10%

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. 🙂

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