1.
For 3-digit numbers, addition is easy when one or both
numbers are multiples of 100 (500 + 767 = 1267) or when
both numbers are multiples of 10 (660 + 310 --> 660 +
300 = 960 and 960 + 10 = 970).
2. To add 3-digit numbers, first add the 100s, then the 10s, then the 1s.
For 425 + 123, first add 425 + 100 = 525. The problem is now simpler, 525 + 23;
keep the 500 in mind and focus on 25 + 23. Add 20 + 20 = 40, then add 5 + 3 to get 48.
The answer will be 548.
3.
We could do 766 + 489 by adding the 100s, 10s, and 1s
digits, but each step would involve a carry. Another way to
do the problem is to notice that 489 = 500 – 11; we can
add 766 + 500, then subtract 11 (answer: 1255).
4.
For mental subtraction, we also work one digit at a time
from left to right. For example 83 – 26, first subtract 83
– 20 = 63.then subtract 6 from 63 ie. 63 - 6, so the
answer is 57.
5. A subtraction problem that would
normally involve borrowing can usually be turned into an easy
addition problem with no carrying.
For 153 – 88, subtract 90, then add back 2: 153 – 90 = 63 and 63 + 2 = 65.
6. For 3-digit numbers, you again subtract the 100s, the 10s, then the 1s.
For 846 – 225, first subtract 200: 846 – 200 = 646. Keep the 600 in mind, then do
46 – 25 by subtracting 20, then subtracting 5: 46 – 20 = 26 and 26 – 5 = 21.
The answer is 621.
7.
Three-digit subtraction problems can often be turned into easy
addition problems. For 835 – 497, treat 497 as 500 – 3.
Subtract 835 – 500, then add back 3: 835 – 500 = 335 and
335 + 3 = 338.
8. For 835 – 467, you first subtract 500 (835 – 500 = 335).
Find the complement of 67 (33) ie 67 is 33 away from 100, and add it to 335:
335 + 33 = 368.