Marginal utility of each unit is found as the change in total utility from the previous unit; here it falls steadily from 15 to 12 to 9 to 6.

Using MU_n = TU_n - TU_(n-1) with TU_0 = 0:

Marginal utility falls continuously — 15, then 12, then 9, then 6 — with each successive unit adding less to total utility than the one before it, and total utility itself keeps rising throughout (it has not yet reached its maximum in this data, since MU has not yet fallen to zero).

Independent verification: summing the MU series, 15+12+9+6 = 42, which matches the given TU of the 4th unit exactly, confirming the MU values are correctly derived.

Cross-check: the differences between consecutive MU values (15-12=3, 12-9=3, 9-6=3) are themselves constant here, which is a coincidence of this particular data set, not a requirement of the law — the law only requires MU to fall, not to fall by a constant amount each time.

A common slip is computing MU as TU divided by the number of units (an average, not a marginal figure) instead of the DIFFERENCE between successive TU values.