# Thread: Sample tossing coin question

1. ## Sample tossing coin question

Dear all, don't know what went wrong with my solution, it doesn't make sense!

Question: Toss a coin twice. What is the probability of getting first toss a head OR second toss a tail?

My solution:
HH, HT, TH, TT
2/4 + 2/4 = 1 ??? Doesn't make sense, why?

Thanks!!!

2. Originally Posted by mak15
Dear all, don't know what went wrong with my solution, it doesn't make sense!

Question: Toss a coin twice. What is the probability of getting first toss a head OR second toss a tail?

My solution:
HH, HT, TH, TT
2/4 + 2/4 = 1 ??? Doesn't make sense, why?

Thanks!!!
The Probability of $\displaystyle P(A \cup B) = P(A)+P(B)-P(A \cap B)$

The probability of the first event is 2/4 the probability of the second event is 2/4 and the probability of both it 1/4 so,

$\displaystyle \frac{2}{4}+\frac{2}{4}-\frac{1}{4}=\frac{3}{4}$

Which, if you look at the possibilities that you wrote out, this solution makes sense.

Hope that helps.