You take 720 and divide by 4 for the 4 weeks. Then you divide by 6 for each of the day and then you reach your answer.
So hmmm let's do the left-hand-side first
now, let's do the right-hand-side then
80% of (Cost) = $68
80% is also 0.8:
0.8 x Cost = $68
dividing each side by 0.8
Cost = 68 / 0.8 = 85 = $85