25% of pupils in a school travel to school by bus. three pupils in the school are selected at random.find the probability that (i)all three travelled (ii)the first two pupils selected do not travel by bus (iii) the first two pupils do not travel by bus but the third one does
hi vicky :)
Hi Vicky a Bernoulli trial can only have two possible outcomes, this is a Bernoulli as you either pick someone who travels by bus or does not travel by bus.
1) Write down the probabilities of the the two possible outcomes
P(Travel by bus) = 0.25 or 1/4 P(Do not travel by bus) = 0.75 or 3/4
(i) (0.25)(0.25)(0.25) = ?
This could also be written (0.25)^3
(ii) Here (a)the first two could travel by bus and the also the third (you worked this out in part i) OR (b)the first two could travel by bus and the third does not.
Work out (a) and (b) and add your answers. The word OR in probability always suggests you add. The word AND suggests you multiply.
Try the question again and tell me how you get on.