secrets-of-mental-math-malestrom

A Little Give and Take: Mental Addition and Subtraction 23
Now try your hand (or head) at 81 37. Since 7 is greater
than 1, we round 37 up to 40, subtract it from 81 (81 40
41), then add back the difference of 3 to arrive at the final
answer:
81 37 41 3 44
40 3
With just a little bit of practice, you will become comfortable
working subtraction problems both ways. Just use the rule
above to decide which method will work best.
EXERCISE: TWO-DIGIT SUBTRACTION
38
23
84
59
92
34
67
48
1. 2. 3. 4. 5.
79
29
63
46
51
27
89
48
125
79
6. 7. 8. 9. 10.
148
86
Three-Digit Subtraction
Now let’s try a three-digit subtraction problem:
958
417
(400 10 7)
This particular problem does not require you to borrow any
numbers (since every digit of the second number is less than the
digit above it), so you should not find it too hard. Simply subtract
one digit at a time, simplifying as you go.