Independent solution

How to solve this Independence question

Setup

Setup

Let E be the emergency-charge event and p the probability of an operating charge. The supplied complement gives Pr(E)=0.75.

Pr(E)=10.25=0.75\Pr(E)=1-0.25=0.75

Model

Model

Use independence in the two-event addition rule, replacing the overlap probability by 0.75p.

0.85=p+0.750.75p0.85=p+0.75-0.75p

Compute

Compute

Equating the union probability to 0.85 simplifies to 0.25p=0.10, hence p=0.40.

0.25p=0.10,p=0.400.25p=0.10,\qquad p=0.40

Answer

Answer

The operating-charge probability is 0.40, corresponding to choice D.

0.40(D)\boxed{0.40\quad\text{(D)}}