Refer to image A box contains 20 resistors Eight of them are

Refer to image

A box contains 20 resistors. Eight of them are rated at 50 ohm and the other twelve are rated at 100 ohm . Two resistors are selected at random. What is the probability that the first resistor is rated at 100 ohm ? What is the probability that the second resistor is rated at 100 ohm , given that the first resistor is rated at 50 ohm ?

Solution

1)

P(first is 100 ohms) = 12/20 = 0.6 [ANSWER]

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2)

If the first resistor is 50 ohms, then 12 out of the remaining 19 is 100 ohms.

Thusm

P(second is 100 ohms|first is 50 ohms) = 12/19 or 0.631578947 [ANSWER]