12378 ÷ 9 = 1375 R3
Alright, let's work through the division of 12378 by 9 step by step.

Step 1:
9 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 (2) of the dividend behind the 1 and repeat the process: 12 / 9

Step 2:
9 goes into 12 1 times with a remainder of 3.
Write down 1 as next digit of of the result. 
Result so far: 1
Subtract 9 from 12 to get 3.
Bring next digit (3) of the dividend behind the 3 and repeat the process: 33 / 9

Step 3:
9 goes into 33 3 times with a remainder of 6.
Write down 3 as next digit of of the result. 
Result so far: 13
Subtract 27 from 33 to get 6.
Bring next digit (7) of the dividend behind the 6 and repeat the process: 67 / 9

Step 4:
9 goes into 67 7 times with a remainder of 4.
Write down 7 as next digit of of the result. 
Result so far: 137
Subtract 63 from 67 to get 4.
Bring next digit (8) of the dividend behind the 4 and repeat the process: 48 / 9

Step 5:
9 goes into 48 5 times with a remainder of 3.
Write down 5 as next digit of of the result. 
Result so far: 1375
Subtract 45 from 48 to get 3.

The final result is 1375 with a remainder of 3.