When total utility is constant, it means marginal utility is

When total utility is constant, it means that the additional satisfaction or usefulness gained from consuming one more unit of a good (marginal utility) is equal to zero. This is because the total utility is the sum of the marginal utilities of all the units consumed. Therefore, if total utility is not changing, it implies that the marginal utility is neither increasing nor decreasing but has reached a point of equilibrium where the additional satisfaction gained from consuming one more unit is zero.