Marginal Revenue is the revenue realised by selling additional units of a product.
Mathematically, Marginal Revenue can be expressed as,

Let old Quantity be
and new Quantity be

Since 500 units were produced first,

And now 100 additional units will be produced, So

Now Change in Quantity produced is

Now we need to calculate for the Change in Total Revenue
We also let the initial total revenue be
.
Initially 500 units were sold for $4 per unit. This implies that,
.
=$2,000
We were given the current total revenue to be $3,000.
This implies that, our change in total revenue
.
Hence our Marginal Revenue


Hence the Marginal Revenue is $10.