Both Mackieman and Bunuel have provided solutions that are spot-on, so I won't rehash any of that here. Instead I want to focus on the brief discussion in the last two posts in this thread - since the subject involves a math "issue" that you WILL see on Test Day.
When it comes to "canceling out" variables, adding or subtracting is just fine (and you should take advantage of those rules to simplify your work).
X + Y = 2X
Y = 2X - X
Y = X
However, you have to be VERY CAREFUL about multiplying or dividing variables away....you might end up accidentally eliminating some of the correct answers....
N^2 = N
This equation has 2 solutions: N = 0 and N = 1.
If you decide to "divide out" the Ns, then look at what happens...
N^2 = N
(N^2)/N = (N)/N
N = 1
You've now "LOST" one of the original solutions (where's N = 0 in this equation?).
Be on the lookout for this issue, since you'll see it at least once on Test Day (look for it in DS questions).
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