Two ways to do the problem, depending on what you are studying.
List the sample space:

{HHHH,HHHT,HHTH,HHTT
 HTHH,HTHT,HTTH,HTTT
 THHH,THHT,THTH,THTT
 TTHH,TTHT,TTTH,TTTT}

Now I will color the ones red which have exactly 3 tails:

{HHHH,HHHT,HHTH,HHTT
 HTHH,HTHT,HTTH,HTTT
 THHH,THHT,THTH,THTT
 TTHH,TTHT,TTTH,TTTT}

There are 4 out of 16, so the probabliity is  or 

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Second way.  Only use this is you are studying binomial probability.
Otherwise ignore it. 

The formula for the probability of getting exactly x successes out of n
independent trials, when the probability of getting one success in one 
trial is p, is



The formula for the probability of getting exactly 3 successes out of 4
independent trials, when the probability of getting one success in one 
trial is , is

















---------------------------

Write as a division:

�

Invert the second fraction and change the division to multiplication:



Factor  out of  and  out of   



Cancel the  into the  getting 3, and cancel the s:

 
            





You can either leave it like that or

distribute the top out and leave it like this:



Or you can write  as 

Here are all the ways a pair of fair dice (number cubes) can be rolled:

(1,1) (1,2) (1,3) (1,4) (1,5) (1,6)

(2,1) (2,2) (2,3) (2,4) (2,5) (2,6)

(3,1) (3,2) (3,3) (3,4) (3,5) (3,6)

(4,1) (4,2) (4,3) (4,4) (4,5) (4,6)

(5,1) (5,2) (5,3) (5,4) (5,5) (5,6)

(6,1) (6,2) (6,3) (6,4) (6,5) (6,6)

That is the sample space.  There are 36
outcomes in that sample space.

Now I will go through and color the ones
red which are either an odd number or a 
multiple of 3)

(1,1) (1,2) (1,3) (1,4) (1,5) (1,6)

(2,1) (2,2) (2,3) (2,4) (2,5) (2,6)

(3,1) (3,2) (3,3) (3,4) (3,5) (3,6)

(4,1) (4,2) (4,3) (4,4) (4,5) (4,6)

(5,1) (5,2) (5,3) (5,4) (5,5) (5,6)

(6,1) (6,2) (6,3) (6,4) (6,5) (6,6)

There are 24 throws out of the 36 that are either odd or are 
a multiple of 3.  

24 successful ways out of 36 possible ways gives a probability
of  which reduces to .

Edwin