<PRE><B>Question 1082101</B>
HHHH
HHHT
HHTH
HTHH
THHH
HHTT
HTHT
HTTH
THHT
THTH
TTHH
TTTH
TTHT
THTT
HTTT
TTTT
Exactly two heads in 4 tosses: 6 ways of having them 4C2
The denominator of the product of the probabilities is always 81.  The numerator numbers change in order but not in product, and they are 2*2*1*1=4.  Multiply by 6 and get 24/81=8/27
exactly 3 heads is 4 * 2*2*2*1/87=32/81
exactly 4 heads is 2*2*2*2/81=16/81
(and 1 head and 0 heads for completeness:
1:2*4/81=8/81
0:1/81
Sum is 81/81</PRE>