# Probabiliy basic

• Apr 9th 2011, 07:38 AM
Nforce
Probabiliy basic
If we flip a coin 6 times. What is the probability to get head exactly one time.

so if we flip a coin one time it's 1/6,
is it for fliping a coin 6 times also 1/6? because the events are independent?

Another one;

In a basket we have 10 white balls and 2 blue balls. Randomly we pick two balls.
What is the probability that two balls are white?

For picking one ball, and to be white is 10/12.
For picking another ball, and to be white is 9/11.

Is it the probability for two, just 9/11 or we have to add together something?

• Apr 9th 2011, 07:46 AM
Plato
Quote:

Originally Posted by Nforce
If we flip a coin 6 times. What is the probability to get head exactly one time.
Another one;

In a basket we have 10 white balls and 2 blue balls. Randomly we pick two balls.
What is the probability that two balls are white?

I am going to play the "back of the book" game with you.
I give you the answer. But you must reply with the reasons.

1) $\dfrac{6}{2^6}$.

2) $\dfrac{10}{12}\cdot\dfrac{9}{11}$
• Apr 9th 2011, 08:04 AM
Nforce
ok, the second answer I understand. 10/12 for the first event and 9/11 for the second for both 10/12 and 9/11 = 10/12 * 9/11

but first I don't. maybe it's not obvious enough for me.

When is going to be exactly 1?
• Apr 9th 2011, 10:33 AM
Plato
Quote:

Originally Posted by Nforce
ok, the second answer I understand. 10/12 for the first event and 9/11 for the second for both 10/12 and 9/11 = 10/12 * 9/11
but first I don't. maybe it's not obvious enough for me.
When is going to be exactly 1?

We are going to have a sting of five T's and exactly one H.
How many ways can this string $TTHTTT$ be arranged?
Now we get $T\text{ or }H$ with equal probability, $\frac{1}{2}$ six times.
• Apr 10th 2011, 09:56 AM
Nforce
yes, now i understand.