average value is the integral divided by the interval length. Thinking in terms of area, the integral is the total area, and a rectangle with that same area would have width of the interval, height the average value
Int(15 - 15e^(-t/2))[0,4]
= 15t + 30e^(-t/2)[0,4]
= (15*4 + 30/e^2) - (0 + 30)
= 60 + 30/e^2 - 30
= 30(1 + 1/e^2)
Average value = 7.5(1 + 1/e^2)
So, the answer is A
