Find firm-s marginal revenue as it increases output

Question

Here is some data on the demand for lettuce:

Price                Quantity

$10                  100

$ 8                   120

$ 6                   140

$ 4                   160

$ 2                   180

(a.) Is demand elastic or inelastic in the $6-$8 price range? How do you know?

(b.) If the table represents the demand faced by a monopoly firm, then what is that firm's marginal revenue as it increases output from 160 units to 180 units? Show all work.