Marginal utility is the change in total utility from consuming one more unit, found using MU_n = TU_n - TU_(n-1).

Marginal utility of the n-th unit is calculated as MU_n = TU_n - TU_(n-1), taking TU_0 = 0.

Marginal utility falls continuously from 10 to 8 to 6, confirming the Law of Diminishing Marginal Utility. It reaches zero at the 4th cup, which is exactly where total utility is at its maximum (24 utils) — this is the point of satiety. Beyond it, the 5th cup actually reduces total utility (from 24 to 20), so marginal utility turns negative, showing the consumer has over-consumed relative to what maximises satisfaction.

Independent verification: re-computing each difference separately — 18-10=8, 24-18=6, 24-24=0, 20-24=-4 — confirms the same marginal utility series, so the answer is verified.

Alternative check: total utility must equal the running sum of the marginal utilities calculated. Sum = 10+8+6+0+(-4) = 20, which matches the TU of the 5th cup (20), cross-confirming the MU series is internally consistent.

Students often forget that TU_0 = 0 and start the MU series from the 2nd cup only, silently dropping the correct MU of the 1st cup (which equals its own TU). Another common error is failing to notice that a FALL in total utility between two rows (24 to 20) must produce a NEGATIVE marginal utility, not a zero or positive one.