Question

There are 6 chemical elements represented by Li, Na, Mg, Rb, Cs, and Fr that

are written on separate pieces paper. You randomly choose 3 elements, one
at a time, without replacement.

Find the probability that the third element is Na.

Enter the correct answer in the box.

Answer 1

The answer is going to be 1/6

Answer 2

1/6

Explanation:

I think this is tricky.

when you randomly choose the 1st two times, it doesn't matter as long as Na was not chosen, otherwise the probability would become ZERO for the 3rd one be Na.

So, after 1st two is removed, there are only 4 left in the pool, so picking Na at the 3rd draw is 1/4 probability.

But, come to think of it, when 1st draw, you have to draw the other 5 out of 6, 2nd draw the 4 out of 5, then 3rd is 1 Na out of the 4.

So, 5/6 x 4/5 x 1/4 = 1/6