GMAT Fast Multiplication

Multiplication by 6

This is not a rule but a trick of convenience. Multiply by 3 followed by 2. You will get to the answer at a much faster pace

6 x 6 = 36

or 6 x 3 = 18 x 2 = 36

Multiplication by 5

It's often more convenient instead of multiplying by 5 to multiply first by 10 and then divide by 2.

For example, 237x5=2370/2=1185

Same way you can also multiply by 50

Multiplication by 4

Replace either with a repeated operation by 2.

For example 124x4=248·2=476

Multiplication by 9:
If you are good with addition and subtraction then this trick would be useful

9 x 11 = 99

or

Place 0 (zero) after the number = 110

Subtract 9 from this new number = 110 - 11 = 99

Multiply by 12: This is a cool trick.

12 x 14 = Lost?

1. Multiply 14 by 10 = 14 x 10 = 140
2. Multiply 2 by 14 = 28
3. Add the numbers = 168

Not cool anymore when you see that 12 x 14 = (10 + 2) x 14

Multiply by 13: Multiply by 3 and add 10
times the original number(Same trick as in the above example) only instead of multiplying by 2 you multiply by 3.

Multiply by 15: Multiply by 10
and add 5 times the original number(Same trick as in the above example)

Multiply by 14: Multiply by 7 and
then multiply by 2

This is not a trick but a rule of convenience

Multiplication by 25

Multiply first by 100 and then divide by 4

For example 37x25=3700/4=1850/2=925.

Multiplication by 8

Replace either with a repeated operation by 2.

For example 124x8=248x4=496x2=992.

Multiplication by 45

Multiply first by 90 and then divide by 2.

For example37x45=37x90/2=3330/2=1665

Multiplication by 55

Multiply first by 110 and then divide by 2.

For example37x55=37x110/2=4070/2=2035

Multiplication by 75

Multiply first by 300 and then divide by 4 .

For example 37x75=37x300/4=11100/4=2775

Multiplication by 125

Multiply first by 500 and then divide by 4

For example 37x125=37x500/4=18500/4=4625

Multiplication by 150

Multiply first by 300 and then divide by 2

For example37x150=37x300/4=11100/2=5550

Multiplication by 175

Multiply first by 700 and then divide by 4

For example37x175=37x700/4=25900/4=6475

Multiplication by 225

Multiply first by 900 and then divide by 4

For example37x225=37x900/4=33300/4=8325

Multiplication by 275

Multiply first by 1100 and then divide by 4

For example37x275=37x1100/4=40700/4=10175

A Great   Time Saver

Q A music shop has a sound system marked at Rs. 14000 on which a sales tax of 12.5% is chargeable. Find the total value a customer would have to pay for the sound system.

     sales tax = 14000x12.5    =    14000x125    = 14x 500 = 7000   =1750

                                100            1000                4                4

                          Rs 14000+1750 =Rs15750

Multiplication by 11

To multiply any figure number by 11 we just put

the total of the two figures between the 2 figures.

32X11

Here first we write 2 of 32 to extreme right , now add up 3+2 =5 , put 5 before 2 and write 3 to extreme left .

Required answer is 352

322x11

Take the 2(Extreme right) from 322 and add up 2&2 of 322 , 2+2=4 , write 4 before 2 Now our partial answer is 42 Again add up 3&2 of 322 .3+2 =5 write before 42 .Now our partial answer becomes ..542 .Put 3 before 5 . Now our required answer is 3542

77 x 11 = 847 This involves a carry figure because 7 + 7 = 14

we get 77 x 11 = 7₁47 = 847

You can do calculations in Mensuration very fast by this tool

As you know value of pi is 22/7 .You can break it as 2x11/7

Ex Find the area of circle of radius of 14 cm

(22/7)x14x14=11x2x2x14=11x56=616square cm

All calculation mentally, no rough work , a great time saver.

Method for multiplying numbers where the first figures are the same and the last figures add up to 10.

42 x 48 = Both numbers here start with 4 and the last figures (2 and 8) add up to 10.

Just multiply 4 by 5 (the next number up) to get 20 for the first part of the answer.

And we multiply the last figures: 2 x 8 = 16 to get the last part of the answer

42 x 48=2016

Method for multiplying numbers where the first numbers add up10 and and

the second number's digits are same 46X55

Here first number's add up is 10 and second number's digits are common i.e 5

Just multiplying last figures of both numbers 6x5 =30 put it in right hand side

Again multiplying first figures of both numbers and add common digit of second number

(4x5)+5 =20+5 =25 put it left hand side

Required answer is 2530 ( If multiplication is in unit's place in first step add zero before it)

Multiplying numbers just over 100.

108 x 109 = 11772

The answer is in two parts: 117 and 72,

117 is 108 + 9 (or 109 + 8),

and 72 is just 8 x 9.

Similarly 107 x 106 = 11342

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