9+10=?

I need to know!

 

21 because

9 is factored into 3 and 3. Keep that in mind.
10 is factored into 5 and 2. Keep that in mind as well.

5*3=15
2*3=6
I got the 5, 3, 2, and 3 from the factoring.
15+6=21

So 9+10=21

 

What kyle said but a more easier way: 9+10 = 21

3*3=9, 5*2=10
3*5=15,3*2=6
6+15=21

 

It's obviously 1.8100399 billion, here is why:

9+10
21+3=4
4(9+10)
4*21=
76
76(9+10)
I have no clue what I'm doing.