probability (1 Viewer)

[Getteral09]

In a set of 30 cards, each one has a number on it from 1 to 30. If 1 card is drawn out, then replaced and another drawn out, find the probability of getting a 3 3 on one card and an 18 on the other card.

Answer is 1/450.

Can someone explain to me how to go about this question? I'm never good at replacement questions.

 

[Riviet]

1/30 x 1/30 (=1/900) since you put the first card back making the total number of cards drawn the second time to be 30. But there are 2 ways a 3 and an 18 can be drawn; the 3 first (then the 18 second) or the 18 first (then the 3 second). Therefore you multiply by two to give 1/450.

 

[Getteral09]

Thanks man! That helped a lot! =D