Answer:
a) Mean = 382.3 psi
Standard deviation = 20.8 psi
b) 392.7 psi
c) P(X<400)=0.8026
Step-by-step explanation:
a) The population mean can be estimated as equal to the mean of the sample, and the population standard deviation can be estimated from the sample standard deviation:


b) We start by searching for the z-value for the 95th percentile. This value is
z=1.645:

Then, the strength value below which 95% of all welds will have their strengths is:

c) We calculate the probability of X being equal or less than 400 as:

For this case, you must use the given expression, taking into account that:
n = number of days.
We have then that for day 21, n = 21.
Substituting we have:
a21 = 100 + (21-1) * 4
a21 = 180
Answer:
she sprints 180 yards on day 21