Although this problem can be solved using the variables themselves, you will be able to make better sense of it and also be able to solve it faster, if you plug in simple values for the variables. Let us see how this can be done.
Let’s take x = 100, p = $10, y= 20 and q = $20. This means, the farmer planted 100 acres of corn at $10 per acre. 20 acres were destroyed by locusts and hence, he sold 80 acres at $20 per acre.
So, cost of farmer = 100 * $10/acre = $1000 and total sales of farmer = 80 * $20/acre = $1600.
Therefore, profit = $600.
Now, all that has to be done is to plug in the values in the options to check which one gives us $600. Clearly, option A is the one that gives $600 when we substitute the values of the variables we had assumed.
Therefore, the correct answer option is A.
Whenever a problem like this presents itself with lots of variables, both in the question and the options, it’s a good idea to plug in simple values rather than going the variables and expressions way. This way, you will also have a way of cross checking if your answers are right.
Hope this helps!