Question

What is the probability of obtaining exactly three heads in four flips of a coin, given that at least two are heads? (Enter the probability as a fraction.)

I keep getting 2/3 but its wrong. Help!!

Answer 1

two are already decide. You have two more flips. What is the chance of getting one heads and one tails out of those two flips ?

You have four possibilities
HH
HT
TH
TT

two of those four work, TH and HT
so I get 1/2