Part a.
Fixed charge for the month: $30
Charge per hour: $0.50 daytime
Charge per hour: $0.25 nights and weekends
Let's say in one month, a person parks for d hours of daytime and n hours of nights and weekends.
The total monthly charge would be
cost = 30 + 0.50d + 0.25n
Now let's see what Trent did.
He parked for 47 hours in one month.
h of the 47 hours are nights and weekends.
Let x = number of daytime hours.
x + h = 47
x = 47 - h
He parked h hours of night and weekends, and he parked 47 - h hours of daytime.
Now we use h for night and weekend hours and 47 - h for daytime hours in the expression above.
cost = 30 + 0.50d + 0.25n
cost = 30 + 0.50(47 - h) + 0.25h
Answer to part a.: 30 + 0.50(47 - h) + 0.25h
Part b.
We are told the actual number of night and weekend hours, which we called h above, is 12. h = 12.
Now we use the cost expression we found in part a. with 12 in for h.
cost = 30 + 0.50(47 - h) + 0.25h
cost = 30 + 0.50(47 - 12) + 0.25(12)
cost = 30 + 0.50(35) + 0.25(12)
cost = 30 + 17.50 + 3
cost = 50.5
Answer to part b.: $50.50