ffirs.indd

128 Speed Math for Kids
Here is the option I prefer, however. Call out the problem backward
as an addition. Th is is probably how you were taught to check
subtractions in school. You add the answer to the number you
subtracted to get your original number as your check answer.
Adding the substitutes upward, we get 6 + 8 = 14.
6 + 8 = 14
Adding the digits, we get 1 + 4 = 5, so our calculation checks.
1 + 4 = 5
Our answer is correct.
I set it out like this:
8 4 6 5 5

• 3 8 9 6 8 +
  4 5 6 9 6
  Test you rself
  Check these calculations for yourself to see if there
  are any mistakes. Cast out the nines to fi nd any errors.
  If there is a mistake, correct it and then check your
  answer.
  a) 5,672 b) 8,542 c) 5,967 d) 3,694
  
  
  • 2,596 – 1,495 – 3,758 – 1,236
    3,076 7,147 2,209 2,458
    They were all right except for b). Did you correct b) and
    then check your answer by casting out the nines? The
    correct answer is 7,047.
    Th is method will fi nd most mistakes in addition and subtraction.
    Make it part of your calculations. It only takes a moment and you
    will earn an enviable reputation for accuracy.