Question 1197952

Hi
3/4 of the boys are equal to 1/3 of the girls. What fraction of the pupils are boys. My son son 9/13 textbook solution  is 4/13. Who is correct. 
Thanks
<pre>Let number of boys be B, and girls, G
We then get: {{{matrix(2,3, (3/4)B, "=", (1/3)G, 3B/4, "=", G/3)}}}
                 9B = 4G ------ Cross-multiplying
                {{{matrix(1,3, 9B/4, "=", G)}}}
Total students: # of boys + # of girls = {{{matrix(1,5, B + 9B/4, "=", 4B/4 + 9B/4, "=", 13B/4)}}}
    Fraction of students that are boys: {{{highlight_green(matrix(1,8, B/(13B/4), "=", B * (4/(13B)), "=", cross(B) * (4/(13cross(B))), "=", highlight(4/13), matrix(1,2, "(the", "textbook)")))}}}</pre>