# Multiplication rule homework

Hi,
Where can I find the solution of the homework from the Multiplication rule video?
"Find the probability of getting a spade on either the 1st turn or the 2nd turn."
Thank you.

1 answers ( 0 marked as helpful)

Hey Harold,
Let's use the following notation:
A - Drawing a spade on the first turn
B - Drawing a spade on the second turn
Now, to clarify, the idea is that we draw a card on the second turn, regardless of whether we got a spade initially. We can apply the Multiplication Law to compute the probability of drawing a spade on the second turn and get the correct answer:
1/4 + 3/4*13/51 = 1/4 + 39/204 = (51 + 39)/204 = 90/204 = 15/34
Note that this is not

*really*P(B), but rather P(B*), because this is only the case we are interested in, the one where we did not draw a spade on the first go. The law of total probability dictates that P(B) = P(B|A)*P(A) + P(B|A')*P(A') = 12/51*1/4 + 13/51*3/4 = 1/4. The idea behind using the Additive Law is a bit more abstract. Since we want to find the probability of getting a spade on either the first or the second turn, we are looking for their union. Thus, P(AuB) = P(A) + P(B) - P(AnB). We already established that P(A) = P(B) = 1/4, so we only need to compute P(AnB). Now, if we get a spade on the first go, we have a 12 in 51 chance of getting a spade on the second turn as well. Thus, P(AnB) = 1/4*12/51. If we plug these numbers into the equation, we get: P(AuB) = P(A) + P(B) - P(AnB) = 1/4 + 1/4 - 1/4*12/51 = 1/2 - 3/51 = (51 - 6)/102 = 45/102 = 15/34. Hope this helps! Best, The 365 Team