How to solve

$(8$ Marks$)$ A concrete has a specified compressive strength of $37\mathrm{.}92$ MPa $.$ Determine the required compressive strength for concrete supplied under the following conditions.

a$.$ A new plant where there is no concrete quality information.

b$.$ An existing plant that has $22$ test results with aSD$=3\mathrm{.}45$MPa.

c$.$ An existing plant with a long history of producing quality concrete with $SD=2\mathrm{.}76$MPa.

d$.$ An existing plant with a long history of producing quality concrete with $SD=4\mathrm{.}14$MPa.