Question

Arsenic is a compound that occurs naturally in very low concentrations. Arsenic blood concentrations in healthy adults are Normally distributed with mean =3.2 micrograms per deciliter (ug/dl) and standard deviation sigma = 1.5. What is the range of arsenic blood concentrations corresponding to the middle 90% of healthy adults?

Answer 1

0.7325 to 5.6675 ug/dl

Explanation:

The middle 90% will be 45% above the mean and 45% below the mean. This means

0.5-0.45 = 0.05 and

0.5+0.45 = 0.95

We use a z table. Look in the cells; find the values as close to 0.05 and 0.95 as we can get.

For 0.05, we have 0.0505 and 0.0495; since these are equidistant from 0.05, we use the value between them. 0.0505 is z=-1.64 and 0.0495 is z=1.65; this gives us z=-1.645.

For 0.95, we have 0.9495 and 0.9505; since these are equidistant from 0.95, we use the value between them. 0.9495 is z = 1.64 and 0.9505 is z=1.65; this gives us z = 1.645.

Now we use our z score formula,

`z=(X-\mu)/(\sigma)`

Our two z scores are 1.645 and -1.645; our mean, μ, is 3.2; and our standard deviation, σ, is 1.5:

`1.645=(X-3.2)/(1.5)`

Multiply both sides by 1.5:

`1.5(1.645)=(X-3.2)/(1.5)* 1.5\\\\2.4675 = X-3.2`

Add 3.2 to each side:

2.4675+3.2 = X-3.2+3.2

5.6675 = X

`-1.645=(X-3.2)/(1.5)`

Multiply both sides by 1.5:

`1.5(-1.645)=(X-3.2)/(1.5)* 1.5\\\\-2.4675=X-3.2`

Add 3.2 to each side:

-2.4675+3.2 = X-3.2+3.2

0.7325 = X

Our range is from 0.7325 to 5.6675.