Answer:
4 days
Explanation:
1) Derive a formula.
Let b = bicycle
Let x = the number of days
A set up equation: b = 6x + 15
2) If she pays 39 to hire a bicycle, substitute 39 into b, then solve for x.
39 = 6x + 15
39 - 15 = 6x
24 = 6x
24/6 = x
4 = x
x = 4
Since x is the number of days, she hired the bicycle for 4 days.