Explanation

You can think of the equation in this question as , where x is an unknown we made up to stand for . It's not an act of fancy math; on the contrary, it's looking at the equation and saying, "10 plus something is equal to 20 minus two times that something." So the difference between 10 and 20 is spanned by a total of three of the something's. So x must be ; 10 and 20 are meeting partway between them by way of 3x. In that case, y, the inverse of our x, is . The textbook method here would be to multiply both sides of the equation by y and isolate the variable. The answer is (C).

Since that went a bit fast, we can expand on a more general point. When you face hard algebra or even easy algebra, you can sometimes make life easier for yourself by creating your own variable, working the problem and/or thinking in terms of that variable, and switching back at the end. For example, if you're faced with , you can introduce the variable and then solve the more familiar and then switch back to x after you have solved for z.

Again, the correct answer is (C).