a math question

[bilibili]

 On Monday, a class has 8 girls and 20 boys. On Tuesday, a certain number of girls joined the class just as twice
that number of boys left the class, changing the ratio of girls to boys to 7 to 4. How many boys left the class on
Tuesday?
(A) 5
(B) 6
(C) 11
(D) 12
(E) 18

 

From Manhattan prep.

Question, which # is bigger? number of boys left the class or number of girls joined the class??

 

Thx

Edited March 4, 2017 by bilibili

[Black Beauty]

@bilibili 

The number of boys who left the class (12 boys) would be bigger since twice as many boys left than girls who joined.

[GREMasterEMPOWERRichC]

Hi bilibili,

This question can be solved in a number of different ways - including TESTing THE ANSWERS. 

We know that there are 8 girls and 20 boys to start, and a certain number of girls joined and TWICE as many boys left the class (leaving us with a ratio of 7 girls for every 4 boys). We're asked for the number of boys that LEFT the class. 

To start, since the number of boys who left is TWICE the number of girls who joined, the number of boys who left MUST be an EVEN number (so we can eliminate Answers A and C immediately).

Let's TEST Answer D: 12 boys

IF... 12 boys left and 6 girls joined, we would have....

8 + 6 = 14 girls

20 - 12 = 8 boys

14:8 = 7:4

This is an exact match for what we were told, so this MUST be the answer.

Final Answer: D

GRE Masters aren't born, they're made,

Rich