A travel agency will plan a group tour for groups of size 25 or larger. If the group contains exactly 25 people, the cost is $300 per person. If each person's cost is reduced by $10 for each additional person above the 25, then the revenue is given by
R(x) = (25 + x) (300 - 10x)
where x is the number of additional people above 25. Find the marginal revenue if the group contains 30 people. Interpret your result.