Question on law of diminishing returns?

Question

How do you know if a firm faces the law of diminishing returns?

ex. Mr. Weldon has a watermelon farm. The marginal cost of the 4th ton of watermelons is 600. The average total cost of producing 6 tons of watermelons = 866.66. The average variable cost of producing 6 tons of watermelon= 700. Does Mr. Weldon face the law of diminishing returns??

Need help!

Answer 1

Unfortunately it is impossible to tell without additional data.

For instance under all-range constant returns MC(q)=AVC(q) but in order to face all-range increasing returns condition MC(q)<AVC(q) always should be true (except first unit there MC=AVC).

For decreasing returns MC(q) >MC(q-1) should be true in some range.