Question

A car park charges a certain rate per hour before 3 pm, and a higher rate per hour

Answer 1

Who's JD?   I'll guess that's money in some country.

This one requires a bit of analysis before writing the equation.  3pm is the time the price changes so

11am to 5 is 4 hours before, 2 hours after, JD18

9am to 7 is 6 hours before, 4 hours after, JD31

OK, let's call the morning rate x and the evening rate y.  

a.

Rate times hours is cost; we have to add up the morning and evening:

4x + 2y = 18

6x + 4y = 31

b.

We multiply the first equation by 2,

8x + 4y = 36

Subtract the second equation,

2x = 36 - 31 = 5

x = 5/2

2y = 18 - 4x

y = 9 - 2x = 9 - 5 = 4

Answer: Our solution is the price was JD 2.50 before 3pm and JD 4 after.

Check:  

2.5(4) + 4(2) = 18 good

2.5(6) + 4(3) = 31 good