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 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
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