Teach me how to divide and show my work

This is how you divide
Set up the equation. On a piece of paper, write the dividend (number being divided) on the right, under the division symbol, and the divisor (number doing the division) to the left on the outside. ...
Divide the first digit. ...
Divide the first two digits. ...
Enter the first digit of the quotient.

answer:

a

step-by-step explanation:

one is 90, and another is 50, so we subtract them to get the answer.

90-50=40, which is the answer.