The demand function for a widget is given by $p=100+\frac{400}{lnx}$ where $p$ is the price of the widget in dollars and $x$ is the number of items in thousands of items and $x>1.$

Find the marginal revenue function. The marginal revenue function is $R^{\text{'}}(x)=(0).$ If $20$ thousand items are currently being made, by how much will the revenue change if we increase production by one thousand items? $(1)($round to $4$ decimal places if rounding is neoded$)$