Addition 109
And for d), you would say 424 plus 30 is 454, minus 2 is 452.
Did you fi nd these easy to do in your head?
What if you have to add 31 to a number? You simply add 30, and
then add the 1. To add 42 you add 40, and then add 2.
Test you rself
Tr y the se :
a) 26 + 21 =
b) 43 + 32 =
c) 64 + 12 =
d) 56 + 41 =
The answers are:
a) 47 b) 75 c) 76 d) 97
You may have thought that all of those answers were obvious,
but many people never try to calculate these types of problems
mentally.
Often you can round numbers off to the next hundred. How would
you do the following problem in your head?
2,351
- 489
You could say 2,351 plus 500 is 2,851 (300 + 500 = 800), minus
11 is 2,840.
Normally you would use a pen and paper to make the calculation,
but when you see 489 as 11 less than 500, the calculation becomes
easy to do in your head.