I will assume that some boys left the club, but the number of girls remained the same.


40% of 90 is  0.4*90 = 36;  so the number of boys initially in the club was 36.


Let x be the number of boys who left the club.


Then the decreased population of the club became  90-x, and there are 36-x boys now.


The condition says   = 0.28,   or


36-x = 0.28*(90-x) 

36-x = 25.2 - 0.28x

36 - 25.2 = x - 0.28

10.8 = 0.72x  ====>  x =  = 15.


Answer.  15 boys left the club.