9 + 10?

Question

I think... it's 21.

Answer 1

Let's say that you already have 9 cookies.  Someone was generous to give you 10 more.  After the amount is added, how many cookies do you have now?  That new amount is the sum.

 

 

Another way to approach this is to use the number line system.  You start with the number 9 on that line.  Now when we add a number, we count to the right on the number line by that number.  When we subtract, we count to the left by that number.  Since we have addition, we count to the right.  We need to add 10, so count 10 units to the right starting from 9.

 

Unit :   1   2    3    4    5    6    7    8   9    10

Total: 10  11  12  13  14  15  16  17  18   19 

 

The value under the unit 10 is 19.  This means that when we add 10 to 9, the total is 19.

 

 

9 + 10 = 19

Answer 2

9 + 10 = 19.

 

I would like to know why you think that sum = 21.