13991 ÷ 6 = 2331 R5
Let's divide 13991 by 6.

Step 1:
6 goes into 1 0 times with a remainder of 1.
Write down 0 as next digit of of the result. 
Result so far: 0
Subtract 0 from 1 to get 1.
Bring next digit (3) of the dividend behind the 1 and repeat the process: 13 / 6

Step 2:
6 goes into 13 2 times with a remainder of 1.
Write down 2 as next digit of of the result. 
Result so far: 2
Subtract 12 from 13 to get 1.
Bring next digit (9) of the dividend behind the 1 and repeat the process: 19 / 6

Step 3:
6 goes into 19 3 times with a remainder of 1.
Write down 3 as next digit of of the result. 
Result so far: 23
Subtract 18 from 19 to get 1.
Bring next digit (9) of the dividend behind the 1 and repeat the process: 19 / 6

Step 4:
6 goes into 19 3 times with a remainder of 1.
Write down 3 as next digit of of the result. 
Result so far: 233
Subtract 18 from 19 to get 1.
Bring next digit (1) of the dividend behind the 1 and repeat the process: 11 / 6

Step 5:
6 goes into 11 1 times with a remainder of 5.
Write down 1 as next digit of of the result. 
Result so far: 2331
Subtract 6 from 11 to get 5.

The final result is 2331 with a remainder of 5.