Compressive strength of concrete cubes made for testing purp

Compressive strength of concrete cubes (made for testing purpose) is assumed to follow a normal distribution with mean strength of 6.000 psi and standard deviation of 1,000 psi. Mean and standard deviation arc representative of the entire population. Consider a sample of (1) 20 cubes and (2) 60 cubes. Problem 1: For each sample estimate the probability that sample average and sample standard deviation fall within 10% of population mean and standard deviation.

Solution

Data:

mean : 6000 psi

standard deviation: 1000 psi

the 10% of mean = 600 so 6000-600 = 5400

a) P=> 5400 psi , n= 20

x=5400 z= (5400-6000)/((1000)/(20)^2) =-2.68 so the probability P=>-2.68 is = 0.00368

I suposse that it is like this.I hope I have help you a little !!