Question 949374
How you go about it is that you follow the words. 
The length of the fence is an unknown, let's call it x:
Arthur painted 2/5 of x
Boris painted 1/4 of x
Chester painted the last 18 feet. 

Now the equation, just follow the problem:
2/5x + 1/4x + 18 = x Let's get rid of the fractions on the left, we need a common denominator:

5*4= 20, this is our common denominator. Multiply both sides times 20:

20(2/5x + 1/4x + 18) = 20x
8x + 5x + 360 = 20x
13x + 360 = 20x 

Subtract 13x, both sides
360 = 7x Divide both sides by 7:
360/7= 7x/7
51.43= x This is the length of your fence, 51.43 feet.

Proof: 51.43(2/5) + 51.43(1/4) + 18= length of fence.
20.57 + 12.86 + 18 = 51.43 We've got the right answer