We group numbers to make them easier to work with. Because we use a base-10 number system, we group numbers in multiples of 10. Ten 1s are the same as one group of 10. One hundred 1s are the same as one group of 100. Most of the time, one group of 10 or one group of 100 is easier to work with than ten 1s or one hundred 1s.
Sometimes, however, numbers are easier to work with if we regroup them -- arrange them into different groups. Lets say, for example, you have to subtract 59 from 87. You line up the tens place and the ones place and get ready to subtract the ones. But you cant subtract 9 from 7 because 9 is bigger than 7. So you have to take one group of 10 (ten 1s) from the tens place. Now you have 7 groups of ten and seventeen 1s. Youve regrouped the 87 into different sized groups. You can subtract 9 from 17; the answer is 8. Then you subtract the digits in the tens place. (Remember you now have only 7 groups of 10.) 7 - 5 = 2. So 87-59=28.
You regroup in some addition problems too. Lets say you want to add 55 and 27. Start with the ones place: 5+7=12. Regroup the 12 as one 10 and two 1s. Put the 2 ones in the ones place, and put the 1 group of 10 in the tens place. Then add the digits in the tens place: 5+2+1=7. So 55+27= 82.
Regrouping Lesson Plans
Regrouping Interactive Activities