Based on the table given of the total benefit per number of shields, the reason why the Knights should not purchase 7 shields if each shield costs $30 is that the marginal benefit is less than the marginal cost.
What should the Knights not purchase 7 shields?
Economic theory dictates that the Knights should purchase shields in such a way that the the marginal benefit should either be larger than the marginal cost, or should be equal to it.
If the marginal cost is larger than the marginal benefit, then the knights should not buy more shields.
The marginal cost of the 7th shield is the cost of $30 while the marginal benefit can be found as:
= Total benefit of 7th shield - Total benefit of 6th shield
= 415 - 410
= $5
Difference:
= 30 - 5
= $25
Buying a 7th shield is gives a marginal benefit that is $25 less than the marginal cost so the knights should not buy it.
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