7. (a) State the Law of Diminishing Marginal Utility: This law states that as a consumer consumes additional units of a good the marginal utility/extra satisfaction derived from each additional unit consumed will eventually decline - Leaving Cert Economics - Question 7 - 2009

Next, for the 3rd unit, the Marginal Utility is calculated as follows:

$ext{Marginal Utility} = ext{Total Utility (3rd unit)} - ext{Total Utility (2nd unit)} = 85 - 65 = 20$

So, the Marginal Utility for the 3rd unit is 20.

For the 4th unit, we calculate:

$ext{Marginal Utility} = ext{Total Utility (4th unit)} - ext{Total Utility (3rd unit)} = 100 - 85 = 15$

Thus, the Marginal Utility for the 4th unit is 15.

For the 5th unit, the calculation is:

$ext{Marginal Utility} = ext{Total Utility (5th unit)} - ext{Total Utility (4th unit)} = 110 - 100 = 10$

This gives the Marginal Utility for the 5th unit as 10.

Lastly, for the 6th unit, we have:

$ext{Marginal Utility} = ext{Total Utility (6th unit)} - ext{Total Utility (5th unit)} = 115 - 110 = 5$

Therefore, the Marginal Utility for the 6th unit is 5.

Diminishing utility sets in after the consumption of the 2nd unit, when the Marginal Utility from consuming the 3rd unit decreases to 20, down from 35 for the 2nd unit.