question Gmat Prep- Practice test

charmingirl8 wrote:The attendees at a certain convention purchased a total of 15000 books. How many of these attendees were female?

1- There was a total of 4000 attendees at the convention
2- The male attendees purchased a average (arithmetic mean) of 3 books each and the female attendees purchased an average of 5 books each.

SOLUTION is C (both) . Can anyone show me how to get the solution? Thanks!

Let the number of male attendees be x and the number of female attendees be y

the first statement says, x + y = 4000 , 1 equation and 2 unknown variables so the equation cant be solved, .. first statement is insufficient.

the second statement says, 3x+5y=15,000, again 1 equation and 2 unknown variable so the equation cant be solved to get the of x and y ..

combine the 2 statements and we have 2 equations and 2 unknown variables(x,y) so they both can be solved to get a unique value for x and y, hence the answer is C ..