Problem $2(10$ points$)$: Wind turbine efficiency

A wind turbine with a $40-m$ diameter rotor produces $287-kW$ output in a $10\frac{m}{s}$ wind.

$(1)$ What is its efficiency?

$(2)$ A wind farm has $100$ such wind turnbines. If the turnbes are arranged by $4-$rotor$-$diameter tower spacing along its rows, and with $7-$diameter spacing between rows $(4D7D).$ What is the power output per unit of land area $(k\frac{W}{m^2})?$ Assume array efficiency is $80\%.$