Answer:
$4
Step-by-step explanation:
Perfectly competitive firms are characterised by:
1) Free entry and exit of buyers and sellers.
2) Large number of buyers and sellers.
3) Existence of identical product.
4) Informations are readily available to the customers.
Marginal revenue(MR) refers to a change in revenue as a result of an additional change in output.
At 100 units output, MR=$400
At 101 units output, MR=$404
Change in MR=$404-$400
=$4
Change in output=101 units-100units
=1 unit
Marginal revenue(MR)= change in revenue/Change in output
Marginal revenue (MR)=$4/1 Unit
MR= $4