Sherri’s babysitter charges a flat fee of $10 and an additional $3 per hour. If last time Sherri owed the babysitter $28, how many hours did the babysitter work?

10+3x = 28

10 + 3 h = 28

3 h = 18
h = 18 / 3

To determine the number of hours the babysitter worked, we need to subtract the flat fee of $10 from the total amount owed, which is $28. This will give us the additional amount that the babysitter earned per hour.

Let's calculate:
Amount earned for additional hours = Total amount owed - Flat fee
Amount earned for additional hours = $28 - $10
Amount earned for additional hours = $18

Now, we need to find out how many hours the babysitter worked by dividing the amount earned for additional hours by the hourly rate of $3.

Let's calculate:
Number of hours worked = Amount earned for additional hours / Hourly rate
Number of hours worked = $18 / $3
Number of hours worked = 6

Therefore, the babysitter worked 6 hours.