The total revenue (in Rupees) from selling $x$ units is $R(x)=3x^2+36x+5$. The marginal revenue when $x=15$ is

1. Marginal revenue $=\dfrac{dR}{dx}=6x+36$. 2. At $x=15$: $6(15)+36=90+36=126$. 3. The trap $90$ forgets the constant term $36$. _Source: syllabus Mathematics Ch 6 "Application of Derivatives", p.5_