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Verified Answer :

C(x) = 0.0004x³ -0.09x² +25x + 339

C'(x) = 0. 0012x² - 0.18x + 25

C' (100) = 12 - 18 + 25 = 19

Marginal cost at 100 units is $19 per unit.


Cost of 101st item: C(101) - C ( 100) 
Plug the value x =100 and x =101, subtract!

C(101) = 0.0004 ( 101)³ - 0.09 ( 101)² + 25 101 + 339 = 2358,0304
C(100) = 0.0004 ( 100)³ - 0.09 ( 100)² + 25
100 + 339 = 2339

C(101) - C ( 100) = 2358,0304 - 2339 = 19,0304