Free Tips to do Mental Math Questions Easy in 2011- Part 4

This is my fourth blog of tips to do well in math problems. Please find some tips to do math questions easy. These tricks are easy to remember to do all math calculations:

Blog-1-Tips to do Mental Math Questions Easy

Blog 2-Free Tips to do Mental Math Questions Easy in 2011

Blog 3-Mental Math Questions Easy

19. 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.

20. Squaring 2-digit numbers that end in 5

If a number ends in 5 then 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.

21. 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.

22. 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

23. 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

24 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, 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

Free Tips to do Mental Math Questions Easy in 2011- Part 2

This is my second part of tips to solve math problems. Please find some tips to math questions easy. These tricks are easy to remember to do all match calculations

First Blog-Free Tips to do math Questions easy

9. 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

2^nd 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

11. 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

12. 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.

13. 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.