Polyester resins reinforced with fiberglass are used to fabricate wall panels of restaurants. It is theorized that adding cement kiln dust (CKD) to the polyester composite will increase wall panel hardness. In a study, hardness (joules [J] per squared centimeters) was determined for three polyester composite mixtures that used a 40% CKD weight ratio. The hardness values were reported as 82, 83, and 78. In your opinion, does using a 40% CKD weight ratio increase the mean hardness value of polyester composite mixtures? Support your answers statistically.

*I already found the correct t-value. t=3.27

I am confused on how to go about getting the p-value which they claim is 0.041.

What steps, equations or tables do I need to find the p-value. I thought I was supposed to take (0.5 - z-score)x2 =p-value if it is a single tail or (0.5 - z-score)x4 =p-value for a two-tail.

If you could break this down into simple steps that would greatly help me. So far none of this is making sense.