Question 547741
a) the sample space is all the possible outcomes.  
If H is heads and T is tails, we can can get these outcomes:
{{H,H},{H,T},{T,T}, {T, H}}

That's the sample space !

b) Looking at our sample space, there is one outcome with H,H (2 heads) and a total of 4 possibilities.  So this probability is {{{1/4}}}

c)  This is similar to the last.  The trick being AT LEAST ONE HEAD means we could have one head, or two heads..... 

Looking at the sample space there is 3 options where this is possible, again a total of 4 possibilities   
So the probability of at least one heads is {{{3/4}}}.

Note: once you get into larger sample spaces an easier way to do it is take the total {{{4/4}}} and subtract the probability of getting no heads {{{1/4}}}. {{{4/4-1/4=3/4}}}  (It's not a coincidence that the two answers are the same..)  


Hopefully this helps.