Given : bag one contain 4 defective and 16 not defective mobile PHONES

bag 2 contains 1 defective and 1 NON defective mobile phones

quality control manager tosses a coin and

selects a mobile phone from bag 1 if it comes up head

otherwise he selects a mobile from bag 2

To Find :  probability that the selected phone is defective​

Solution:

Probability of Head = 1/2

Probability of other wise = 1 - 1/2  = 1/2

selects a mobile phone from bag 1 if it comes up head

bag 1 contain 4 defective and 16 not defective mobile phones

Total phone = 4 + 16 = 20

Probability of defective from bag 1 = 4/20   = 1/5

Over all probability of defective from bag 1 = (1/2)(1/5)

= 1/10

selects a mobile phone from bag 2 if it does not comes up head

bag 2 contain 1 defective and 1 not defective mobile phones

Total Phones = 1 + 1 = 2

Probability of defective from bag 2 = 1/2  

Over all probability of defective from bag 2 = (1/2)(1/2)

= 1/4

probability that the selected phone is defective​  = 1/10  + 1/4

= ( 2 + 5)/20

= 7/20

7/20 will be the probability that the selected phone is defective​

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