The ratio of number of girls to the number of boys in a class is $5 : 4$ . If there are $25$ girls in the class, then the number of boys in the class is

15

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20

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30

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40

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Solution

The correct option is B $20$
$I f t h e r a t i o o f t h e b o y s t o g i r l s i s 5 : 4, t h e n w h a t i s t h e n u m b e r o f b o y s a n d g i r l s .$

$F i r s t, y o u w o u l d a d d 5 a n d 4, b e c a u s e t h a t i s t h e n u m b e r s i n t h e r a t i o .$

$5 + 4 = 9$

$L e t t o t a l n o o f s t u d e n t s = 45$

$T h e n, y o u w o u l d d o \frac{45}{9}, a n d t h a t w o u l d l e t y o u f i n d o u t h o w m a n y s t u d e n t s i s r e p r e s e n t e d b y e a c h n u m b e r i n t h e r a t i o .$

$\frac{45}{9} = 5$

$Y o u w o u l d t h e n m u l t i p l y 5 t o b o t h, 5 a n d 4, t o f i n d o u t t h e n u m b e r o f b o y s t o g i r l s .$

$5 (5 : 4)$

$25 : 20$

$T h e r e a r e 25 g i r l s a n d 20 b o y s .$

$T o c h e c k, 25 + 20 i s e q u a l t o 45, t h e n u m b e r o f s t u d e n t s i n t h e c l a s s, a n d i f y o u s i m p l i f y 25 : 20, i t s i m p l i f i e s t o 5 : 4.$

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