Shortcut for Multiplication by 11

 

Multiplication by 11

Multiplication by 11 is easy! To multiply by a 2-digit number add the two digits and place the sum in between!

25 x 11 = 275

2   (2+5)   5
2     7      5


31 x 11 = 341

3  ( 3+1)  1
3      4     1


57 x 11 = 627 (you need to carry the 1)

5   ( 5+7)    7
5 _{← 1} 2    7
6    2     7

             
 What about a 3or 4-digit number? Can you figure out what's going on here?                        

253 x 11 = 2783

2 (2+5) (5+3) 3
2     7     8     3

1857 x 11 = 20427

1 (1+8) (8+5) (5+7)  7
1    9 _←1 3_←1 2    7
2  0  4  2  7