Question

The security system in a house has two units that set off an alarm when motion is detected. Neither one is entirely reliable, but one or both always go off when there is motion anywhere in the house. Suppose that for motion in a certain location, the probability that detector A goes off and detector B does not go off is 0.25, and the probability that detector A does not go off is 0.35. What is the probability that detector B goes off?

Answer 1

Answer:

Probability that detector B goes off is '0.615'

Step-by-step explanation:

Given that:

1) Probability that detector A goes off and detector B does not go off is 0.25.

2)Probability that detector A does not go off is 0.35.

3)Probability that detector A  goes off is (1-0.35)=0.65

Assuming that

Probability that detector B goes off is 'p' Hence the probability that detector B does not goes off  is (1-p)

Thus the probability that detector A goes off and detector B does not go off is  product of the individual probabilities

$P(E)=0.65\times (1-p)\\\\0.65\times (1-p)=0.25\\\\0.65-0.65p=0.25\\\\0.65-0.25=0.65p\\\\\therefore p=\frac{0.40}{0.65}=0.615$

Probability that detector B goes off is '0.615'