Asked : Total number of people Surveyed

(1) Six-tenths of those who received the survey responded.

6/10(Total) responded, We cannot find the total. Not Suff

(2) Of those who received the survey, 42 responded.

We cannot find the total. Not Suff

Combining 1 & 2

6/10(Total)=42

We can calculate Total number of people Surveyed

Hence C
_________________

Target question: How many people received a certain survey?

Statement 1: 6/10 of those who received the survey responded.
This statement is clearly insufficient to answer the target question with certainty
Statement 1 is NOT SUFFICIENT

Statement 2: Of those who received the survey, 42 responded.
This statement is clearly insufficient to answer the target question with certainty
Statement 2 is NOT SUFFICIENT

Statements 1 and 2 combined
Let T = the total number of people who received the survey

From the information in statement 1, we can write: The number of people who responded to the survey = (6/10)T
Since statement to tells us that 42 people responded to the survey, we can write: (6/10)T = 42
At this point, we need only recognize that we COULD solve this equation for T, which means we COULD answer the target question with certainty
As such, the combined statements are SUFFICIENT
Answer: C

ASIDE: Let's solve the equation "for fun"
Take: (6/10)T = 42
Multiply both sides of the equation by 10 to get: 6T = 420
Divide both sides by 6 to get: T = 70
This means that 70 received the survey

Answer: C

Cheers,
Brent
_________________