From what I'm reading from this question, there's one answer.
13 + 5 * 1 = 18
Unless maybe I'm missing something?
Answer:


Step-by-step explanation:
Given

Solving (a): Write as inverse function

Represent a(d) as y

Swap positions of d and y

Make y the subject


Replace y with a'(d)

Prove that a(d) and a'(d) are inverse functions
and 
To do this, we prove that:

Solving for 

Substitute
for d in 




Solving for: 

Substitute 5d - 3 for d in 

Add fractions



Hence:

3x + 4 = 2x + 2
3x - 2x = 2 - 4
x = -2
one solution
2 pounds ---- 100%
x pounds -----18%
x=(2*18)/100=36/100=0.36 pounds