In a bilingual school there is a class of 21 pupils. In this class, 15 of the pupils speak spanish as their first language and 12 of these 15 pupils are Argentine. The other 6 pupils in the class speak english as their first language and 3 of these 6 pupils are argentine.
A pupil is selected at random from the class and is found to be argentine. Find the probability that the pupil speaks spanish as his/her first language.
The Attempt at a Solution
A = speaks spanish as first language
A' = speaks english as first language
B = is Argentine
B' = is English
so P(A|B) = (15/21) x (12/15) = 0.8
(15/21)(12/15) + (6/21)(1/2)
Does this seem right?