GMAT Prep Probability Question

[ssy]

If a coin has an equal probability of landing heads up or tails up each time it is flipped, what is the probability that the coin will land heads up twice on 3 consecutive flips?

0.125
0.25
0.375
0.50
0.666

0.375

I solved this by writing down all the possible outcomes and counting the number of times a coin could land heads up twice out of the total number of outcomes.

I.e.

HHH
TTT
THH
HHT
HTH
TTH
THT
HTT


Is there an easier, less time-consuming way of solving this?

[samirpandeyit62]

I think method is good enough, it depends upon how u work out the choices

u may use the formula for Binomial Probabilty here if u like

which is nCr * p^r * q ^n-r

n = total nos of trails
r =total nos successfull trials reqd
p = P of success
q= P of failure

https://en.wikipedia.org/wiki/Binomial_probability

[camitava]

ssy,
My approach to solve this problem will be -
Total favorable cases : for first coin = 2 for second coin = 1
So Total favorable cases = 3

Again total cases = 2 * 2 * 2 = 8
So probability = 3/8.

[cvs]

Amtiva,

Can you explain how the 1st coin will have 2 possible out come..?
(Total favorable cases : for first coin = 2 for second coin = 1 )

Thanks