probs- coins

[demoskopico]

I have some difficulties with coins...can someone tell me a good general rule? thanks..

example:

If a certain coin is flipped, the probability that the coin will land heads is 1/2. If the coin is flipped 5 times, what is the probability that it will land heads up on the first 3 flips and not on the last 2 flips?

A. 3/5
B. 1/2
C. 1/5
D. 1/8
E. 1/32

[kevin]

Remember that when a coin is tossed repeatedly, the outcome (i.e. heads or tails) on any toss is independent of that on the previous toss. Also, if a fair coin is tossed, Pr(H)=Pr(T)=1/2

Thus Pr(HHHTT)= (1/2)^5 = 1/32

How about this: What is the probability that, when a fair coin is tossed five times, exactly 2 tails are obtained?