Each week John earns X dollars per hour for the first 40

You're reasoning is right on and illustrates how to apply the most powerful tool in your DS arsenal: the number of equations vs number of unknowns rule.

The rule states:

Looking for applications of this rule on DS questions will allow you to quickly and confidently select the correct answer without doing much math - we love it when that happens!

For example, here's how we could have analyzed this question:

Q stem: 3 variables (X, Y, amount earned), 1 equation. What do we need to solve? Either 2 more distinct linear equations (that introduce no new variables) or 1 special equation that eliminates both undesired variables.

(An important corollary to the rule is that if you're only solving for part of the system, you may not need the standard number of equations; similarly, if you're solving for a relationship among variables instead of the actual values of variables, you can often solve with fewer equations.)

1) one distinct linear equation, no new variables: insufficient.
2) one distinct linear equation, no new variables: insufficient.

Together: 2 distinct linear equations, no new variables: sufficient - choose (C).

Note that we didn't need to translate the equation from (2) - we just needed to be sure that it was different from the other two equations, didn't introduce any new variables and was linear (i.e. no exponents other than 1 attached to our variables).

I agree with Stuart. This is a very powerful tool. Though, beware. In some questions, you may need only one statement.
e.g.
A sold two kinds of ice creams: Vanilla for 52 cents each and Chocolate for 58 cents each. How many vanilla ice creams did he sell?
1. He sold a total of 9 ice creams.
2. Total value of ice creams sold was $4.92.

Here answer is (B). Check out my blog for an explanation of these type of questions. (Post - Integral solutions of Ax + By = C)
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