Answer: For

Nick does not has enough money to follow this recommendation.
Step-by-step explanation:
Given: A self-service car wash charges $4 for the initial 5 minutes plus an additional $0.75 for each minute after that.
Let y be the cost for car wash for m minutes then
such that at m= 5 , y=4 which is the cost for car wash (dollars) for first 5 minutes.
Also, A car enthusiast magazine recommends spending at least 15 minutes to properly wash a car.
For

But Nick has $10 which is less than $11.5, so he does not has enough money to follow this recommendation.
There are 20 orange skittles and 10 red skittles since 10 20 is 2 Times more than 10.
A exponential function would represent it nicely f(x)=b*a^x
Reason being a decrease of %
a = 1+r
r = -25% = -0.25
And b being what the original cost of the book
f(x)=85*0.75^x
Where f(x) is the price after x amount of previous owners