A) 100
B) 120
C) 80
D) 150
Correct Answer: B
Solution :
The ratio of the number of boys to girls is\[7:5\]. We make this part to part ratio to part to whole ratio by using the property. \[a:b\Rightarrow \frac{a}{a+b}:\frac{b}{a+b}\] \[=\frac{7}{7+5}=\frac{7}{12}\] \[\therefore \]Ratio of the boys to the total students \[=\frac{7}{7+5}=\frac{7}{12}\]and the ratio of the girls to the total students \[=\frac{5}{7+5}=\frac{5}{12}\] To get the answer, we would first find out the actual number of boys and girls in the school. For this, we multiply the total number with their respective ratios. \[\therefore \]Number of boys \[=\frac{7}{12}\times 720=7\times 60=420\]and Number of girls\[=\frac{5}{12}\times 720=5\times 60=300\] Now, we need to obtain the boys to girl ratio as\[1:1\]. For this, the number of boys and girls should be equal. This can be obtained by adding \[420-300=120\] girls in the school.
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