A printing machine, capable of only printing the 10 digits, prints the right digit 12% of the time. Whenever the machine prints erroneously, all wrong possibilities are equally likely.

If a random key is pressed twice and a same digit comes out both times, what is the probability that it was the correct one?

(In reply to How often is it wrong? by Caz)

We do get a wrong number 88% of the time, and a second identical wrong number 88/9% of the time.

This gives a total probability of .88 * .88/9 = 0,0860444

The probabilty of 88% already considers the possibility of 9 different wrong numbers, therefore multiplying by 9 is wrong.

Gregor

Posted by Gregor on 2006-08-14 08:46:48