Answer:
The value is 
Step-by-step explanation:
From the question we are told that
The mean is 
The standard deviation is 
The safest water level is between 7.2 and 7.8
Generally the probability that the selected pool has a pH level that is not considered safe is mathematically represented as

Here

Generally 
So
=>
From the z-table the probability of (Z < -1.5) and ( Z <1.5) are

and

So
So

=> 
Let the number of nickels found be n, the number of dimes be d and the number of quarters be q.
i) "you have twice as many quarters as dimes and 42 coins in all."
means that 2d=q, and n+d+q=42
we can reduce the number of unknowns by substituting q with 2d:
n+d+q=42
n+d+2d=42
n+3d=42
We can write all n, d and q in terms of d as follows:
there are n=42-3d nickels, d dimes and q=2d quarters.
ii) In total there are $6.60 dollars,
1 nickel = 5 cent = $0.05
1 dime = 10 cent = $0.1
1 quarter = 25 cent = $0.25
thus
(42-3d)*0.05 + d*0.1 +2d*0.25= $6.60
2.1 - 0.15d+0.1d+0.5d=6.60
2.1+0.45d=6.6
0.45d=6.6-2.1=4.5
d=4.5/0.45=10
iii)
so, there are 10 dimes, 2d=2*10=20 quarters and 42-3d=42-3*10=12 nickels.
Answer: 10 dimes, 20 quarters, 12 nickels
Quotient refers to division:
(8/v)² ⇒ (8/v)(8/v) = 8*8 / v*v = 64/v²
When you raise a fraction to a power, you multiply the fraction by itself according to the power raised.
Then, do the usual steps of multiplying fractions.
1) multiply numerators.
2) multiply denominators
3) simplify fraction produced.