F - 3/4 = 5/6
Add 3/4 to both sides to get
F = 5/6 + 3/4 = 19/12
3.250 liters of softener because you have to jump three times according to the ladder method.
First lets remove the initial fee

150 is how much she is charging you per hour.
If each hour is 55, lets divide 150 by 55.

She charged you for 2.7 hours (almost 3) of service