Question

a group of students attend a math club half the students are boys and 4/9 of the boys have brown eyes what fraction of the group are boys with brown eyes

Answer 1

4/9 * 1/2 = 4/18 = 2/9

So, the boys with brown eyes are 2/9 of all the students that attend the math club

Answer 2

From the whole group half the club are boys. 
fraction of boys from the whole class = 1/2
from this fraction, 4/9th of the boys have brown eyes
therefore fraction of boys who have brown eyes 
= 4/9 of 1/2
the 'of' function is used to multiply 
$\frac{4}{9} * \frac{1}{2}$
when multiplying fractions numerators are multiplied by numerators and denominators multiplied by denominators
$\frac{4*1}{9*2} = \frac{4}{18}$
4/18 can be simplified by dividing the whole number by a common number
4/18 divided by 2 = 2/9
the answer is 2/9
2/9 of the club are boys with brown eyes