a. To solve for x, we can multiply both sides of the equation by 5 to get rid of the fraction:

5 * (a + 13) / 5 = 12 * 5
a + 13 = 60

Next, we can subtract 13 from both sides to isolate a:

a = 60 - 13
a = 47

Therefore, the solution to the equation (x + 13) / 5 = 12 is x = 47.

b. Similar to the previous equation, we can multiply both sides of the equation by 5 to eliminate the fraction:

5 * (x - 20) / 5 = 40 * 5
x - 20 = 200

Next, we can add 20 to both sides to isolate x:

x = 200 + 20
x = 220

Therefore, the solution to the equation (x - 20) / 5 = 40 is x = 220.

c. To solve for x, we can multiply both sides of the equation by 18:

18 * (x - 36) / 18 = 12 * 18
x - 36 = 216

Next, we can add 36 to both sides to isolate x:

x = 216 + 36
x = 252

Therefore, the solution to the equation (x - 36) / 18 = 12 is x = 252.