S1 Probability June 2005

7. In a school there are 148 students in Years 12 and 13 studying Science, Humanities or Arts subjects. Of these students, 89 wear glasses and the others do not. There are 30 Science students of whom 18 wear glasses. The corresponding figures for the Humanities students are 68 and 44 respectively.

A student is chosen at random.

Find the probability that this student

(a) is studying Arts subjects,
(4)
(b) does not wear glasses, given that the student is studying Arts subjects.
(2)

Amongst the Science students, 80% are right-handed. Corresponding percentages for Humanities and Arts students are 75% and 70% respectively.

A student is again chosen at random.

(c) Find the probability that this student is right-handed.
(3)
(d) Given that this student is right-handed, find the probability that the student is studying Science subjects.
(3)

I've had a look at the mark scheme and I understood it, however, how would i know that drawing a sample space diagram (I think thats what its called?), is better than a tree diagram. Originally I started off with a tree diagram but it got confusing

Reply 1

Yeahwhat

U mean a two-way table. A space diagram is useless here. A Venn diagram is also useful

Reply 2

Yeahwhat

Watch this : https://www.youtube.com/watch?v=-r_rl0wREzA

Reply 3

doglover123

Yeah thats what i meant :colondollar:

I watched it - in his words "its a good idea to draw a table", I still don't get why using a tree diagram wouldn't work, in the real exam thats what i would have done