Since we’re trying to find minutes, concert all known information to minutes
1 hr 15 mins = 75 mins
1 hr 30 mins = 90 mins
Next, calculate how many total minutes Gage has skated in the first 8 days
75(5) + 90(3) = 645 mins
Create an equation to find the average of Gage’s minutes of skating. Add up all the minutes and divide by the total amount of days and set equal to 85, the average we are trying to get.
(645 mins + x mins)/9 days = 85
Solve for x
645 + x = 765
x = 120
So, in order to have an average of an 85 minute skate time, Gage would need to skate 120 minutes on the ninth day.