A certain type of thread with mean tensile strength of 78.3kg is manufactured by

machine A. Machine B produces a certain type of thread with mean tensile

strength of 77kg. A sample size of 20 threads is taken from machine A and found

that the standard deviation tensile strength of 5.6kg while another a sample size

of 17 threads is taken from machine B and found that the standard deviation

tensile strength of 4kg. Assume the both population standard deviation are

unknown but equal. What is the probability that

a.

the sample mean tensile strength produced by machine A is bigger than the

sample mean tensile strength produced by machine B?