probability

the number of students in a class is more than 30 but less than 40. the number of girls is exactly20% more than the numbers of boys
(a) find the number of girls in the class
number of girls = x
numbers of boys = x/(1+20%) = x/1.2
x + x/1.2 = 11x/6
30 < 11x/6 < 40
180 < 11x < 240
16.36.. < x < 21.81...
x = 17, 18, 19, 20 or 21
but x = 18, 其他唔要, 因為其他數令 numbers of boys 有點數
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(b) if a student is selected at random from the class,what is the probability of getting of getting a girl?
numbers of boys = 15
probability = 18/(18+15) = 6/11
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(c) if two students are selected at random ,what is the probability of getting of getting one boy and one gril?
(18/33)(15/32) + (15/33)(18/32) = 45/88

the number of students in a class is more than 30 but less than 40. the number of girls is exactly20% more than the numbers of boys

(a) find the number of girls in the class

number of girls = x

numbers of boys = x/(1+20%) = x/1.2

x + x/1.2 = 11x/6

30 &lt; 11x/6 &lt; 40

180 &lt; 11x &lt; 240

16.36.. &lt; x &lt; 21.81...

x = 17, 18, 19, 20 or 21

but x = 18, 其他唔要, 因為其他數令 numbers of boys 有點數

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(b) if a student is selected at random from the class,what is the probability of getting of getting a girl?

numbers of boys = 15

probability = 18/(18+15) = 6/11

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(c) if two students are selected at random ,what is the probability of getting of getting one boy and one gril?

(18/33)(15/32) + (15/33)(18/32) = 45/88