<PRE><B>Question 875106</B>
Q:
One semester the ratio of boys to girls in a class was 4 : 5
The next semester, one girl left the class and the ratio of boys to girls was then 6 : 7
How many boys were there in the class? 
A:
The number of boys is 4k and the number of girls is 5k.
So {{{(4k)/(5k - 1) = 6/7}}}, cross-multiply
30k - 6 = 28k
30k - 28k = 6
2k = 6, divide both sides by 2
k = 3
Therefore, 4k = 4(3) = 12 boys</PRE>