Idiot\'s Guides Basic Math and Pre-Algebra

16 Part 1: The World of Numbers

Our 15 tens can be broken up into one group of 10 tens and one group of 5 tens. The group of 10

tens makes 1 hundred. You’re going to pass that 1 hundred over to the hundreds place, to the left,

and just put the 5, for the remaining 5 tens, in the tens digit place. You put the 5 in the tens place

of the answer, and place a small 1 above the hundreds column to remind yourself that you’ve

passed 1 hundred along. In common language, you put down the 5 and carry the 1.

When you add the hundreds digits, you add on that extra 1. Five hundreds + 3 hundreds + the

extra 1 hundred from regrouping = 9 hundreds.

As you finish the addition in the other columns, you’ll find that each gives you just one digit, so

no further regrouping or carrying is needed.

A problem may not need regrouping at all, like our first example, or just once, like this example,

or many times, or even every time. Say you want to add 9,999 and 3,457.

In the ones place, 9 + 7 = 16. Put down the 6 and carry the 1. In the tens, 9 + 5 + 1 you carried

= 15. Put down the 5 and carry the 1. In the hundreds, 9 + 4 + 1 you carried = 14. Put down the

4 and carry the 1. In the thousands, 9 + 3 + 1 you carried = 13.



43,572

12,381

53

1



43,572

12,381

1

953



43,572

12,381

1

55,953



9,999

3,457

13,456

111

CHECK POINT

Find each sum.

1. 48 + 86


2. 97 + 125


3. 638 + 842

4. 1,458 + 2,993

5. 12,477 + 8,394