Final answer:
Define x as the amount you owe your sister. The original debt is $100, and you've paid $58, so the equation is 100 - 58 = x. Solving it gives x = 42, hence you still owe $42.
Step-by-step explanation:
To represent the amount of money you still owe your sister after paying her $58 out of $100, you can define a variable to represent the remaining amount. Let's say x represents the amount of money you still owe. The original amount owed is $100, and you have paid $58, so the equation would be as follows:
100 - 58 = x
When you solve this equation, you subtract 58 from 100 to find the value of x:
100 - 58 = 42
x = 42
Therefore, you still owe your sister $42.