Researchers at a pharmaceutical company have found that the effective time duration of a safe dosage of pain relief drug is normally distributed with the mean 1.5 hours and standard deviation 0.8 hours for a patient selected at random
a) what is the probability that the drug will be effective between 1 and 3 hours after taken
?
b) what is the probability the drug will be effective for 4 hours or less?

a)

b)

Step-by-step explanation:

1) Previous concepts

Normal distribution, is a "probability distribution that is symmetric about the mean, showing that data near the mean are more frequent in occurrence than data far from the mean".

The Z-score is "a numerical measurement used in statistics of a value's relationship to the mean (average) of a group of values, measured in terms of standard deviations from the mean".  

2) Part a

Let X the random variable that represent the duration of a safe dosage of pain relief drug, and for this case we know the distribution for X is given by:

Where and

We are interested on this probability

And the best way to solve this problem is using the normal standard distribution and the z score given by:

If we apply this formula to our probability we got this:

And we can find this probability on this way:

And in order to find these probabilities we can find tables for the normal standard distribution, excel or a calculator.  

3) Part b

We are interested on this probability

And the best way to solve this problem is using the normal standard distribution and the z score given by:

If we apply this formula to our probability we got this:

And we can find this probability on this way:

the expression (1.26)2t reveals the approximate rate of increases in the number of plants semiannually.

hope this : )