problems about probablity

Does getting three tails refer to the occurrence of the three fair coins showing three tails at once?

2013-05-19 11:47:59 補充：
I assume getting three tails refer to the occurrence of the three fair coins showing three tails at once.

Then probability of getting three tails = (1/2)^3 = 1/8

Number of occurrences = np = 40*(1/8) = 5

2013-05-19 20:52:41 補充：
5 is the Expected Number of occurrences, which is an estimation

There are multiple interpretation of this question.

First interpretation: Getting exactly three tails:
(40C3)/(2^40)
=0.00000000898580765351653099...
or 8.98580765351653099... × 10^(-9)

Second interpretation: Getting at least three tails:
1-[(40C2+40C1+40C0)/(2^40)]
=0.999999999253304849844425916671752929688...
or 9.99999999253304849844425916671752929688... × 10^(-1)

Third interpretation: Getting three tails in a row:
38/2^40
=0.000000000034560798667371273040771484375...
or 3.4560798667371273040771484375... × 10^(-11)

2013-05-19 19:27:30 補充：
nCr = (n!)/[(n-r)!r!]