Please give me the formulas to use as well.

2. CarMD reports that the cost of repairing a hybrid vehicle is falling even while typical repairs on conventional vehicles are getting more expensive. The most common hybrid repair, replacing the hybrid inverter assembly, had a mean repair cost of $ 3,927 in 2012. Industry experts suspect that the cost will continue to de-crease given the increase in the number of technicians who have gained expertise on fixing gas- electric engines in recent months. Suppose a sample of 100 hybrid inverter assembly repairs completed in the last month was selected. The sample mean repair cost was $ 3,800 with the sample standard deviation of $ 500 a. Is there evidence that the population mean cost is less than $ 3,927? (Use a 0.05 level of significance.) b. Determine the p- value and interpret its meaning