Learn Some Cool Secrets To Do Mental Math Questions Easy

Please find some tips to math questions easy. These tricks are easy to remember to do all match calculations.

Everybody knows that if we want to multiply with any number with 10 or 100 you just add one or two 0 after that number. E.g. if you multiply 39 with 10, you just add one 0 to 39 to become 390.

These tricks are easy to remember to do all math calculations:

 

Question 1

How to multiply by 12?

First multiply the number by 10. To that product, add twice the number to reach the answer.

Example: 45 X 12

First Multiply: 45 x 10 = 450.

Then Multiply: 5 x 2 = 90.

Then Add 450 + 90 = 540

2nd trick

As above, first multiply the number by 10. What you do then is add half of that product to the product to get the answer.

Example: – 32 X 15

First Multiply: 32 X 10 = 320.

Then Divide: 320 ÷ 2 = 160

Then Add: 320 + 160 = 480

 

Question 2

How to multiply by 1.5, 2.5, 3, 5 etc.

This is made a lot easier if you first double the 1.5, 2.5 or other multiplier and then halve the other number, before multiplying the two products. (It's a lot easier when multiplying even numbers than odd numbers).

Example A:  – 3.5 X 16

First Double: 3.5 X 2 = 7.

Then Halve: 16 ÷ 2 = 8.

Then Multiply: 7 X 8 = 56

Example B. – 2.5 X 13

First Double: 2.5 X 2 = 5.

Then Halve: 13 ÷ 2 = 6.5.

Then Multiply: 5 X 6.5 = 32.5

Question 3

 How to divide by 1.5, 2.5, 3, 5 etc.

This technique is very similar to number 4. What you do hear is double both the divisor and the dividend, before dividing.

Example 1:. – 28 ÷ 3.5

First Double: 28 X 2 = 56

Then Double: 3.5 X 2 = 7.

Then Divide: 56 ÷ 7 = 8

Example 2 – 17 ÷ 1.5

First Double: 17 X 2 = 34.

Then Double: 1.5 X 2 = 3.

Then Divide: 34 ÷ 3 = 11.333 (the answer). As you can see, the even numbers work a lot easier than the odds.

Question 4

How to do percentages?

It is common knowledge that 'percent' literally means 'for every hundred'. However, this quick way to solve any percentage problem is not so widely known. All you do is multiply the two numbers and then move the decimal point two spaces left (divide by 100). Remember to ignore any decimal points and zeroes and re-affix them at the end.

Example: 10% of 20

First Multiply: 10 X 20 = 200

Then Move the decimal point two spaces left = 2.0

Also it’s useful to know that you can always spin percentages and arive at the same answer, e.g. 4% of 52 is the same as 52% of 4.

Question 5

 Multiplying by a power of 2

To multiply a number by 2, 4, 8, 16, 32, or some other power of 2 just keep doubling the product as many times as necessary. If you want to multiply by 16 then double the number 4 times since 16 = 2x2x2x2.

15x16: 15x2 = 30. 30x2 = 60. 60x2 = 120. 120x2 = 240.

23x8: 23x2 = 46. 46x2 = 92. 92x2 = 184.

54x8: 54x2 = 108. 108x2 = 216. 216x2 = 432.

Question 6

 Squaring 2-digit numbers that end in 5

If a number ends in 5 than its square always ends in 25. To get the rest of the product take the left digit and multiply it by one more than itself.

35x35 ends in 25. We get the rest of the product by multiplying 3 by one more than 3. So, 3x4 = 12 and that's the rest of the product. Thus, 35x35 = 1225.

To calculate 65x65, notice that 6x7 = 42 and write down 4225 as the answer.

85x85: Calculate 8x9 = 72 and write down 7225.

Question 7

Multiplying by 9, or 99

Multiplying by 9 is really multiplying by 10-1.

So, 9x9 is just 9x (10-1) which is 9x10-9 which is 90-9 or 81.

Let's try a harder example: 46x9 = 46x10-46 = 460-46 = 414.

One more example: 68x9 = 680-68 = 612.

To multiply by 99, you multiply by 100-1.

So, 46x99 = 46x (100-1) = 4600-46 = 4554.

Multiplying by 999 is similar to multiplying by 9 and by 99.

38x999 = 38x (1000-1) = 38000-38 = 37962.

Question 8

 How easy to calculate %

If you need to calculate 15% of 50.Work out 10% (divide the number by 10) – then add that number to half its value and you have your answer:

15% of $50 = (10% of 50) + ((10% of 50) / 2)

$5.00 + $2.50 = $7.50

Question 9

 Easy way to do tough multiplication

If you have a large number to multiply and one of the numbers is even, you can easily subdivide to get to the answer:

64x250 is the same as:

32x500 is the same as:

16x1000 is the same as:

8x2000 is the same as:

4x4000=16,000

Question 10

 Easy Subtracting from 1,000

To subtract a large number from 1,000 you can use this basic rule: subtract all but the last number from 9, and then subtract the last number from 10:

1000

-648

step1: subtract 6 from 9 = 3

step2: subtract 4 from 9 = 5

step3: subtract 8 from 10 = 2

Answer: 352

11. To multiply any number by 4 all you do is double it, and then double it again.

Example: 16 X 4.

First Double the 16: 16 X 2 = 32

Then Double the 32: 36 X 2 = 64

 

12. To multiply any number by 5, just divide the number by 2, and then re-affix any necessary decimal points or zeroes.

Example: 56 X 5.

First Divide: 56 ÷ 2 = 28.

Then Make sense? 18 is obviously too small to be the answer to 56 X 5. It's clear that the answer is somewhere in the 100s. So you add a zero to the 18 and you have the correct answer – 280.

 

13. To multiply by 9

An easy one - Simply multiply the number by 10, and then subtract the number itself from the result.

Example: 18 X 9

First Multiply: 18 X 10 = 180.

Then Subtract: 180 - 18 = 162

14. To multiply any two-digit number by 11.

First, write the number, leaving a space in between the two digits. Then insert the sum of the two digits in between the two digits themselves.

Example: 54 X 11.

First Write down the two digit number, leaving a gap between the two digits: 5 _4.

Then add the two digits: 5+4 = 9.

Then Insert the sum between the 3 and the 4, which gives you the answer – 594.

You will have to ‘carry’ when the sum of the two digits exceeds 9.

Example:  48 X 11.

First Write 4 _8.

Then add the two digits: 4 + 8 = 12.

Then because the two digits total more than 9, you must insert the 2, which gives you 428,

Then ‘carry’ the 1 (add it to the 4), giving the answer - 528.

 

15. To multiply any number by 25,

To do this, just divide the number by 4, and then add any necessary decimal points or zeroes.

Example: 36 X 25.

First. 36 ÷ 4 = 9

Then. 9 and 90 both seem too small to be the correct answer to 36 X 25.

Then. Add two 0 to the 9 giving the correct answer - 900.

 

16. Divide number by 25

Here, you multiply the number by 4, and then add any necessary decimal points or zeroes.

Example:  500 ÷ 25.

First ignore the zeroes and multiply 5 X 4 = 20.

17. To multiply any two-digit number by 99

First, subtract 1 from the number to obtain the left hand part of the answer. Then, subtract the number from 100 to get the right hand part. Sounds a bit complicated, I know. Look at the example below and it will become clear.

Example: 15 x 99.

First Subtract: 15 – 1 =14. (Left- hand part of the answer).

Then Subtract. 100 – 15 = 85. (Right-hand part of the answer).

Then Put both parts together: 1,485 is the answer.

 

18. To multiply any two-digit numbers from 11 to 19.

Example. – 18 x 16.

First Take the whole of the higher number: (18) and add to the second digit in the smaller number: 18 + 6 = 24.

Then Add a zero to the end of the answer (multiply by 10): 240.

Then Multiply the second digits of the original sum: 8 X 6 = 48 and add it to the answer from step 2: 240 + 48 = 288

Example 2. – 430 X 9

First Ignore the zero and multiply: 43 X 10 = 430.

Then Subtract: 430 - 43 = 387

Then Make sense? It's obvious that 387 cannot be the answer as it's lower than the number you are multiplying. Add a nought and you have the correct answer - 3,870