Regrouping

Question

Explain why the number of tens decreases after regrouping? That is the writing and reasoning question given. The only other problems on the page are 426+235, 339+156, 408+58 and 147+424.  It is second grade math.

Hope that helps.

Answer 1

Assume a two-digit subtraction with a single regrouping, such as 24 - 7.  The number 24 means 2 tens plus 4 ones; that is 10+10+1+1+1+1.  When you line up the problem into a ones column and a tens column, it looks like this:

 

tens | ones

 

   2     4

-        7

  ---------

 

You start in the ones column and take 7 away from 4.  But 7 is bigger than 4.  You can't take 7 things when you only have 4.  So, you have to "borrow" one of the 2 tens from the tens column and add it to the 4 in the ones column to make it 14.  That leaves only one ten in the tens column (as a note, one ten plus fourteen ones (10+14) still equals 24, so the number has not changed)

 

 tens | ones
  ¹ 2    14
-          7
   ---------

    1    7

 

I hope that helps some.  If not, ask again.