You deposit $50 in your savings account. one week later, you withdraw $20. write each amount as an integer

When you deposit $50 into your account, you add it to the amount already in your account. (Some people oversee this error and assume the account has a balance of 0.)

So you add 50 to x, our variable, and then subtract 20 from it because we withdrew $20.

So the amount remaining is x+30.

However, the question asks for every amount to be written as an integer. Let's say that 50 is a, 20 is b, and the original amount is x.
The equation would be x+a-b.