As per first graph,
The number of students in Collage A = Total number of students x 21%
The number of students in Collage A = 3500 x 21/100
The number of students in Collage A = 35 x 21 = 735
As per second graph,
The total number of girls from College A = Total number of girls x 23%
The total number of girls from College A = 1800 x 23/100
The total number of girls from College A = 18 x 23 = 414
Number of boys from college A = Total number of students in Collage A - The total number of girls from College A
Since, number of boys from college A = 735 - 414 = 321
Required percentage = 100 x Number of Boys from Collage A / Total number of boys in collage A
Required percentage = 100 x 321/735
Required percentage = 2140/49 = 43.67%