Concept: The question tests on the ability of framing and solving equations.


Let us assume that the regular charges that John actually had was x dollars, Let the estimate of the time that he intended for this particular job = y hours, 
=>x y =36
But however, there is a little bit of a change because the job took four hours longer. 
So, its y+4 now and it no longer is x$ ,rather (x-2).
 (x -2)(y+4)=336 
=>(x-2) (y+4) = x y(Both equal 336)
=>x y + 4x -2y – 8 = x y 
=>  y = 2x-4 …….(1)
Now plug in the options (for y) and check where you get x y =336.
At Y=24 (option B) x= y+4/2(from 1) = 14 
 X y= 14 x 24 = 336 .  Hence Option B.