How to determine the MU in economics?

Question

I currently have a table like so:

Hours spent on Activity X | Total Utility

1. 120
2. 220
3. 300
4. 360
5. 396
6. 412

I know that Marginal Utility is calculated use slope formulate (delta Y / delta X). I am asked to calculate the MU for 5 hours: So in this case, $delta X = 5 - 1 = 4$ and $deltaY = 396 - 120 = 276$ Then, MU = $deltaY/deltaX = 276/4 = 69$

Next I am supposed to calculate the MU per dollar, which is determined by: $deltaY/deltaX * 1/Price$ . The question states the price of activity X is $5/hour.

Then, I would have $MU per dollar = 69/5 = 13.8$ This gives me 13.8 dollars as the MU per dollar, however, the answer to this question is 7.2 dollars. Can anyone please explain where I've gone wrong?

Answer 1

Actually you are calculating MU wrong $$\frac{\Delta Y}{\Delta X}=\frac{Y_5-Y_4}{X_5-X_4}=\frac{396-360}{5-4}=36$$ $$\Rightarrow \text{MU}_\text{per dollar}=36/5=7.2$$