A bag contains (2n + 1) coins.


A bag contains (2+ 1) coins. It is known that of these coins has a head on both sides whereas the rest of the coins are fair. A coin is picked up at random from

the bag and is tossed. If the probability that the toss results in a head is 31/42, determine the value of n.


Given, n coins are two headed coins and the remaining (n + 1) coins are fair.

Let E1: the event that unfair coin is selected

E2: the event that the fair coin is selected

E: the event that the toss results in a head


P(E1) = n/(2n + 1) and P(E2) = (n + 1)/ (2n +1)

P(E/E1) = 1 (As it’s a sure event)

P(E/E2) = ½

Therefore, the required value of n is 10.

Leave a comment


Click here to get exam-ready with eSaral

For making your preparation journey smoother of JEE, NEET and Class 8 to 10, grab our app now.

Download Now