Problem Solving

Hi,

I have compiled some speedy calculation techniques, you ppl might find it useful

Multiplications:

*) To multiply a number with 5 multiply with 10 and divide it by 2. For example 550*5 multiply by 10 = 5500 now half of it (divided by 2) is 2750.

*) To multiply a number by 25 multiply by 100 and divide by 4 for example 555*25 multiply by 100 = 55500 and now divide by 4 which is 13875

*)To multiply a number by 11. This is best explained by an example. Lets say 7469*11. You need not perform all the traditional multiplication just assume 0s at both the ends of 7469 hence now the number will appear as 074690. Now starting from right start adding last two digits. Hence the product should be 82159

*) To multiply a number by 12. Lets take the same number 7469. Now as in the above case assume 0s hence number will appear as 074690. Now starting from right double each digit and add to the right digit so the number will appear as 89628.

*) To multiply by 13 rule is same but instead of doubling you will triple the digits.

*) Squares of numbers which is one more than or less than the known square can be written this way. For example take 312=302+31st odd number =900+61= 961

Note: to find xth odd number multiply x with 2 and subtract 1 from it. For example 30th odd number is 30*2-1 =59

Finding unit digit: Lets put it in example again

What is the unit digit of 12*2*3*17*13*16. It will take 5-7 seconds to say that unit digit is 2

How? here it is. when you are finding unit digit just consider unit digit of each multiplication, In the above case u multiply 2 of the number 12 with the next number 2 we get 4 now u multiply this with next number 3 we get 12 just take unit digit which is 2 we multiply this with 7 of the number 17 we get 14 take unit digit 4 and multiply by 3 of the number 13 we get 12 take unit digit 2 and multiply with 6 of the number 16 we get 12 hence unit digit is 2.

Let me know if you find these useful. I am in the process of compiling more shortcuts and will also be compiling some quick trickies in other chapters which will be useful for GMAT. You can also find this and much more in my blog. Link to my blog is mentioned in my signature

Thanks
Quant-Master

although simple and strightforward, i seem to miss these shortcuts under time pressure. having it grouped on a list sure does help. thanks again.

Hi,

Yes this is useful... thank you