# Conditional probability -plz help

• May 2nd 2010, 01:08 PM
dee2020
Conditional probability -plz help
1. Jean travels by bus or taxi. The probability that she takes bus on one day and she goes by taxi the next day is 0.4, the probability that she takes taxi on one day and she goes by bus the next day is 0.7. Given Jean takes bus on Monday, find the probability that she takes a taxi on Wednesday?

2. Two coins are flipped. One is fair with head on one side and tail on other. The second coin is tricky one - it has 2 tails on both sides. She picks one at random and flips it.
(a) Probability it lands heads up?
(b)it lands tails up, probability she picked up fair coin?

Any help or hint much appreciated

Dee
• May 2nd 2010, 01:19 PM
rubic
could you check the first question again!

2. Two coins are flipped. One is fair with head on one side and tail on other. The second coin is tricky one - it has 2 tails on both sides. She picks one at random and flips it.
(a) Probability it lands heads up?

(1/2)*(1/2) = 1/4

(b)it lands tails up, probability she picked up fair coin?

(1/2)/(3/4) = 2/3
• May 2nd 2010, 02:57 PM
dee2020

could you please explain 2(b) bit.

what if the question 2(a)asks to find out the probability it lands tails up? how do we work this out?
• May 2nd 2010, 03:39 PM
rubic
(a) Probability it lands heads up?
try to draw a tree diagram

if we choose tricky coin there is no chance of head
otherwise
(probability of choosing fair coin)x(probability of landing head given that you picked fair coin)

(1/2)*(1/2) = 1/4

(b)it lands tails up, probability she picked up fair coin?

(probability of landing tail given that you picked fair coin)/
((probability of landing tail given that you picked fair coin)+(probability of landing tail given that you picked tricky coin))

(1/2)/(3/4) = 2/3
• May 4th 2010, 02:22 PM
dee2020