Selecting a sample at random

[tragicallylost]

Hi. I am having a problem with this question:

Samples of emissions from three suppliers are classified for conformance to air-quality specifications. The results from 100 samples are summarized as follows:

yes no
supplier1: 22 8
supplier2: 25 5
supplier3: 30 10

Let A denote the event that a sample is from supplier 1, and let B denote the event that a sample is selected at random. What is P(B)?

The answer given in class was .77. I dont understand how this was acheived. Are they assuming that event A happens first? I know P(A) = .30. I thought that since there are 100 samples, any one could has the outcome of .01 of being drawn.

I'm lost. Please help if possible. Thanx!

 

[tkhunny]

Let B denote the event that a sample is selected at random. What is P(B)?

Let's add just one word that probably should have been included in the problem statement.

Let B denote the event that a CONFORMING sample is selected at random. What is P(B)?

 

[tragicallylost]

Thank you so much! I never would have figured that out!