Question 1117553
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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  {{{(36-x)/(90-x)}}} = 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 = {{{10.8/0.72}}} = 15.


<U>Answer</U>.  15 boys left the club.
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