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Bunuel
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Active GMAT Club Expert! Tag them with @ followed by their username for a faster response.
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Hey guys,

So the formula for the average here would be:

The sum of all the scores / The total number of students = 45

We don't know the numerator or the denominator

When we don't know the value of something, often using a variable in its place is a good idea:

x = The total # of students
y = The sum of all the scores

\(\frac{y}{x} = 45\)

This means that y = 45x

We are told that 90 students had their score lowered by 30 points from 80 to 50. As a result, the overall average came down to 40

How do we use this information to answer the question?

Well if 90 students had a score decrease of 30, how would that affect the total sum of all the scores?

90 * 30 = 2,700

The new sum is y - 2,700

Here is the new equation:

\(\\
\frac{\\
(y - 2,700)}{x} = 40\)

This means that:

40x = y - 2,700


We can substitute for y from the first equation:

40x = 45x - 2,700

2,700 = 5x

x = 540

The total number of students is 540

The answer is C
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The answer should be C 540
Let x be the total number we need to find
miscalculate by 90*( 80-30)/x= 45-40=5=> x= 2700/5=540
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