To the left is a table illustrating the simple computation of MRP for a firm which operates in a perfectly competitive output market (so marginal revenue = price). The column labeled N represents the number of units of the input, say number of workers, Q gives total output. MP (Marginal Product), is the additional output generated by hiring an additional worker.

The MRP is simply the addition to revenue that can be attributed to the additional worker, so it is simply MP X P. Notice that it is declining as more workers are hired because MP is falling.