If 20 students in a class failed an exam, the probability of a student selected at random getting passed in an exam is 0.6. The number of students in the class is = ___ .

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Solution

Let the number of students in a class be $x$ .

The probability of students passing an exam = 0.6

Number of students who failed an exam = 20

Number of students who passed an exam $= x - 20$

Probability of a student passing an exam $= \frac{Number of students who passed an exams}{Number of students in a class}$

$0.6 = \frac{x - 20}{x}$

$0.6 x = x - 20$

$0.4 x = 20$

$x = 50$

The number of students in the class is 50.

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