Question

Some boys and girls are having a car wash to raise money for a class trip to China. Initially $40\%$ of the group are girls. Shortly thereafter two girls leave and two boys arrive, and then $30\%$ of the group are girls. How many girls were initially in the group?

$4$

$6$

$8$

$10$

$12$

Answer: C

Concepts:percentage invariant

Difficulty rating: 1330

Solution:

Since two girls leave while two boys arrive, the total group size is unchanged. The drop from $40\%$ to $30\%$ girls corresponds to the two girls who left.

So those two girls are $10\%$ of the group, meaning the group has $20$ people. The initial number of girls was $40\%$ of $20,$ or $8.$

Thus, the correct answer is C.

2007 AMC 12B Problem 10

Answer: C

Difficulty rating: 1330

Solution:

Problem 10 in Other Years