Simple maths tricks for competative exams

To find out if a number is divisible by seven:
Take the last digit, double it, and subtract it from the rest of the 
number. 
If the answer is more than a 2 digit number perform the above 
again.
If the result is 0 or is divisible by 7 the original number is also 
divisible by 7.

Example 1 )   259
9*2= 18.
25-18 = 7 which is divisible by 7 so 259 is also divisible by 7.

Example 2 ) 2793
3*2= 6
279-6= 273

now 3*2=6
27-6= 21 which is divisible by 7 so 2793 is also divisible by 7 .

Now find out if  following are divisible by 7

1) 2841
2) 3873
3) 1393
4) 2877

TO FIND SQUARE OF A  NUMBER BETWEEN 40 to 50
Sq (44) .

1) Subtract the number from 50 getting result A.
2) Square A  getting result X.
3) Subtract A from 25 getting result Y
4) Answer is xy

EXAMPLE 1 : 44
50-44=6 
Sq of 6 =36
25-6 = 19
So answer 1936

EXAMPLE 2 : 47
50-47=3
Sq 0f 3 = 09
25-3= 22
So answer = 2209

NOW TRY To Find Sq of 48 ,26 and 49

TO FIND SQUARE OF A 3 DIGIT NUMBER :

LET THE NUMBER BE XYZ

SQ (XYZ) is calculated like this

STEP 1. Last digit =  last digit of SQ(Z)
STEP 2. Second Last Digit = 2*Y*Z + any carryover from STEP 1.
STEP 3. Third Last Digit 2*X*Z+ Sq(Y) + any carryover from STEP 
2.
STEP 4. Fourth last digit is 2*X*Y + any carryover from STEP 3.
STEP 5 . In the beginning of result will be Sq(X) + any carryover 
from Step 4.

EXAMPLE :

SQ (431)

STEP 1. Last digit =  last digit of SQ(1) =1
STEP 2. Second Last Digit = 2*3*1 + any carryover from STEP 
1.= 6
STEP 3. Third Last Digit 2*4*1+ Sq(3) + any carryover from STEP 
2.= 2*4*1 +9= 17. so 7 and 1 carryover
STEP 4. Fourth last digit is 2*4*3 + any carryover  (which is 1) . = 
24+1=25. So 5 and carry over 2. 
STEP 5 . In the beginning of result will be Sq(4) + any carryover 
from Step 4. So 16+2 =18.

So the result will be  185761.

If the option provided to you are such that the last two digits are 
different, then you need to carry out first two steps only , thus 
saving time. You may save up to 30 seconds on each 
calculations and if there are 4 such questions you save 2 
minutes which may really affect UR Percentile score.

Read more: http://www.careers-india.com/2009/09/21/math-shortcuts-for-competitive-examinations-%e2%80%93-part-3/#ixzz2TfAi6rmn

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