Question
The ratio of the number of girls to the number of boys
in a school of 990 students is 2 : 7. If 20 boys left the school, find how many new girls may be admitted so that the ratio of number of girls to the number of boys may change to 3 : 5.Solution
Ratio of number of girls to the number of boys = 2:7 Sum of the terms of the ratio = 2 + 7= 9 
The number of girls in the school =    x 990 = 220  and the number of boys in the school =    x 990  = 770 Let the number of new girls admitted be x, then the number of girls become (220 + x) After 20 boys leave the school, the number of boys become (770 -20) i.e. 750 According to given,  220 + x = 3 150  220 + x  = 450    x = 230
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