As per first graph,
The number of students for course D = Total number of students x 20%
As per second graph,
The number of girls for course D = Total number of girls x 30%
Number of boys in course D = The number of students for course D - The number of girls for course D
Number of boys = ( 1200 x 35/100 ) - ( 800 x 30/100 ) = 420 - 240 = 180
Number of girls = 800 x 30/100 = 240
Since, required ratio = 180 : 240 = 3 : 4
