Answer:
The answer is 
Step-by-step explanation:
If we assume that people cannot taste a difference between bottled water, then the probability of identifying tap water is 0.5
Thus, P(identify tap water)=0.5
The probability that at least 8 of the 9 people identify the tap water correctly is the sum of the probabilities
- 8 of 9 people identified correctly or
- 9 of 9 people identified correctly
Since P(identify tap water)=0.5 each probabilities are the same and equal to
=
So we have 
=
Let's call 'it' x. 1/2 is equal to one third of x, so we could say that 1/3x = 1/2
Now we just have a simple equation to solve:
1/3x = 1/2
x = (1/2) / (1/3)
Dividing by a rational number (such as 1/3, which is expressed in fraction form) is the same as multiplying by its reciprocal (the reciprocal of a fraction is itself when the numerator and denominator have been swapped). Therefore
x = (1/2) / (1/3) = (1/2) * 3 = 3/2 = 1.5
To check this answer, test the statement. Half is a third of x, where x=1.5:
1.5 / 3 = 0.5 = 1/2
81ft(yd/3ft)*111ft(yd/3ft)*2in(yd/36in)=55.5 yd^3
So 55 1/2 cubic yards of topsoil are needed.
B. Plug in the numbers and use common sense.
