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

[xjinaloveskangin]

|-------------+------+------+------+------+-------+
|       Grade |  9th | 10th | 11th | 12th | Total |
|-------------+------+------+------+------+-------+
| Algebra 1   |  864 |  192 |  150 |   63 |  1269 | 
| Geometry    |  175 |  771 |  332 |  196 |  1474 |
| Algebra 2   |    2 |  141 |  427 |  215 |   785 |
| Advan. math |    0 |    4 |   87 |  247 |   338 |
|-------------+------+------+------+------+-------+
| Total       | 1041 | 1108 |  996 |  721 |  3866 |
|-------------+------+------+------+------+-------+

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

----------------------

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!
________________________________
Edited by stapel -- Reason for edit: formatting table.

 

[pka]

For #1 you must use the basic counting rule.

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

 

[xjinaloveskangin]

^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

 

[stapel]

can someone help me one the others please?

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

Eliz.

 

[galactus]

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".