a) To calculate the overall mean, you must take the total wages divided by the total number of workers. What Stacy is doing is calculating the average of the means, which is incorrect as both the total wages and the quantity of workers are both different (or she's just using the wrong method of calculation)
b) First, we find the total wages of the 56 workers : 42 x 9.5 + 14 x 13.5 = 588 (pounds)
So, the overall mean is : 588 : 56 = 10.5 (pounds/h)