If I ate two sevenths of a pumpkin pie and Sophie ate 1/3 of the same pie, how much of the pumpkin pie would be left?

First step here is to find out the current area of the garden:  
12(20) = 240
the problem states that we have to increase each side by the same amount (let's call that value x) to have the new area of 360 sqft. we can express that in a mathematical equation as follows:  
(12+x)(20+x) = 240 

here you are given an option to use mathematical manipulation to find your x or simply use all the choices and substitute it to x. which ever choice gives 360 is the answer.