I need some probability help please...thank you :)

1) What is the probability of selecting a student who is in geometry or is in the 11th grade?

My answer: (1474/3866) + (322/996) = 0.7146

2) What is the probablility of selecting an algebra 2 student who is not in the 11th grade?

My answer: (358/785) = 0.456

3) What is the probability of selecting an advanced math student given that you will be selecting a senior?

My answer: (247/338) = 0.7308

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I think I am doing these wrong and the answers don't seem right...yet I'm not sure another way on how to do them. Please help asap!! Thank you so much!
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Edited by stapel -- Reason for edit: formatting table.

For #1 you must use the basic counting rule.

\(\displaystyle \begin{array}{rcl}
P(G \cup 11) & = & P(G) + P(11) - P(G \cap 11) \\
& = & \frac{{1474}}{{3866}} + \frac{{996}}{{3866}} - \frac{{332}}{{3866}} \\
\end{array}\)

^how do you know when to use that and when not to?

also, can someone help me one the others please? thanks so much xDDDD

xjinaloveskangin said:

can someone help me one the others please?

Click to expand...

Don't the others work in pretty much the same way...?

Eliz.

For #1, the operative word is OR:

\(\displaystyle \L\\\underbrace{\frac{1474}{3866}}_{\text{geometry}}+\overbrace{\frac{996}{3866}}^{\text{11th grade}}-\underbrace{\frac{332}{3866}}_{\text{both}}=\frac{1069}{1933}\)

#2 looks OK.

#3: \(\displaystyle \L\\\frac{247}{721}\)

You have "what's the probability of selecting a senior given they're an adavnced math student".