Checking answers to addition

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Addition checking

Of course we can check our additions the same way as we do as many times as we want, but quite often we will make the same mistake. As such even if we get the same answer the second time it may not be correct. This checking is only to be used when you have spare time after you have completed your test. Do not check your answer this way if you still have other problems to do for your test or examination.

Let us say we want to add the following numbers:
24123 + 43523 + 32126. We work out the problem as in the left column below and
check the answer as shown below on the right.

                    Check
   24123        2+4+1+2+3  = 12
   43523        4+3+5+2+3  = 17
   32126        3+2+1+2+6  = 14
............                             .....
   99772                              43
...........                             ......

Reducing our answer 99772 to a single number as in multiplication we get:

    9+9+7+7+2  = 34   now 3+4 = 7

Our check number  43 now 4+3 =7

Our answer 99772 is therefore correct.

Let us do another example just to make sure we all understand.

                      Check
  12541           1+2+5+4+1  = 13
  68922           6+8+9+2+2  = 27
  33442           3+3+4+4+2  = 16
...........                                ....
114905                                  56
..........                                .......

Reducing our answer to a single number we get :

1+1+4+9+0+5 = 20    Now 2+0 = 2

Our check number 56 now 5+6 =11 and 1+1 = 2

Our check number is the same as our answer when reduced to a single digit.
So our original answer is correct.

Next week we'll make the above even simpler.

Luqman Michel

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