Mental math can be used for addition, subtraction, multiplication and division of numbers. You can quickly rearrange complex problems into simple ones using some simple mental maths rules.

 

Mental Addition

Normally we add numbers from right to left. This is easy when you are using pen and paper. However in mental maths you can do it in the following ways:

Example 1:

57 + 16

You can start with the first column (tens place) of both numbers and calculate 50 + 10 = 60.

Then calculate the sum of the numbers in the units position: 7 + 6 = 13.

Now add the two together to get 73.

 

Example 2:

293 + 480

You can add 7 to 293 to get 300.

Then add 300 and 480 to get 780.

To get the final answer you must now subtract 7 from the result to get 773.

 

Mental Subtraction

Mental subtraction can be done in the same way.

Example 1:

58 - 16

You can start with the first column and calculate 50 - 10 = 40.

Then calculate 8 - 6 = 2.

Now add the two together to get the answer 42.

 

Mental Multiplication

You can be very good at mental maths if you memorise multiplication tables.

  • One of the simplest rules is that when you multiply a number by 10, place a zero at the end of the number to get the answer.
  • Similarly for multiplication by 100, 1000... add that many zeros at the end of the number to get the answer.
  • When you multiply a number by 5, the result will always have a 5 or a 0 in the units (ones) place.
  • When you multiply by 15, you can multiply the given number by 10 and then add half of that result to the original result to get the final answer.

Example: 15 * 4 can be calculated by multiplying 10 * 4 + half of (10*4) = 40 + 20 = 60.

  • If you have to multiply by 99, 999, 9999 etc. you can multiply the given number by 10, 100, 1000... and subtract from the result.

            Example:

            7 * 99 = 7*100 - 7 = 700 - 7 = 693

 

Mental Division

When dividing two numbers try to first reduce them as much as possible.

Example:

360 ÷ 60

You can first reduce both numbers by 10

Hence, 36 ÷ 6 = 6

 

Try the following:

  • Take any 4 digit number.
  • Rearrange the digits to form the smallest and the largest possible number.
  • Subtract the smaller number from the larger number.
  • Now repeat the process for the result. You will eventually reach the number 6174. This number 6174 is called Kaprekar’s constant.

                        Example: 5674

                        7654 - 4567 = 3087

                        8730 - 0378 = 8352

                        8532 - 2358 = 6174

 

Questions for Mental Maths

 

Q1. Use mental math to find the answers:

A1.

a) 435, b) 58, c) 1690, d) 396, e) 978

 

Q2. Solve the following using mental math.

 

A2.

a) 313, b) 46, c) 778, d) 254, e) 116

 

Q3. Find each product mentally.

A3.

a) 100, b) 200, c) 240, d) 2400, e) 2400

 

Q4. Find each quotient mentally.

A4.

a) 4, b) 4, c) 80, d) 60, e) 6

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