This is a classic weighted average question.
The rule you need to know is this:
If you know the "value" of each of two or more groups (in this case it's the grade), you only need to know the fraction/percent/ratio of each of the groups relative to the others to get the weighted average. You do NOT need to know the actual number of people.
Here, it is enough to know that there are twice as many people in group 1. You could use Statement 2 to find the actual number of people in each group, but that is not relevant.
You can find the answer with formula:
(Fraction/Percent in Group 1) * (Value of Group 1) + (Fraction/Percent in Group 2) * (Value of Group 2)
In this case:
[spoiler](2/3)(4) + (1/3)(3) = x. Statement 1 is Sufficient.[/spoiler]
Statement 2 is not sufficient because knowing the difference does not tell you the fraction or percent. It could be 5 people and 1 person (5/6 in group 1) or it could be 7 people and 3 people (7/10 in group 1).
Greg Michnikov, Founder of GMAT Boost
GMAT Boost offers 250+ challenging GMAT Math practice questions, each with a thorough video explanation, and 100+ GMAT Math video tips, each 90 seconds or less.
It's a total of 20+ hours of expert instruction for an introductory price of just $10.
View sample questions and tips without signing up, or sign up now for full access.
Also, check out the most useful GMAT Math blog on the internet here.
