With the support of a graph, explain clearly the behavior of Marginal Utility; and explain when Total Utility is at its maximum.

1. **Describe the Relationship:**
MU_n = \frac{\Delta TU}{\Delta Q}
Where $Q$ is the quantity consumed.

2. **Law of Diminishing Marginal Utility:**
- Initially, each additional unit of the good consumed provides a positive but decreasing Marginal Utility.

3. **Graphical Representation:**
- Total Utility (TU) curve starts to rise but increases at a decreasing rate and eventually flattens out.
- Marginal Utility (MU) curve starts high and then decreases, intersecting the x-axis.

4. **Determine the Maximum Total Utility:**
- Total Utility is at its maximum when the slope of the Total Utility curve is zero. This occurs where Marginal Utility is zero.

5. **Mathematical Condition:**
- Setting $MU_n = \frac{\Delta TU}{\Delta Q} = 0$ gives us the maximum point of Total Utility.

\text{Total Utility is at its maximum when Marginal Utility (MU) equals zero.}