40 ÷ 3 = 13 R1
We're going to perform division on 40 with 3 as the divisor.

Step 1:
3 goes into 4 1 times with a remainder of 1.
Write down 1 as next digit of of the result. 
Result so far: 1
Subtract 3 from 4 to get 1.
Bring next digit (0) of the dividend behind the 1 and repeat the process: 10 / 3

Step 2:
3 goes into 10 3 times with a remainder of 1.
Write down 3 as next digit of of the result. 
Result so far: 13
Subtract 9 from 10 to get 1.

The final result is 13 with a remainder of 1.