Answer
See below
Work Step by Step
Let $x$ be the price of the drum and $n$ be the number of drums.
Given: $n=50\\x=120$
When $n$ increases by $4$, $x$ decreases by $5$
Obtain: $n=50-\frac{4}{5}(x-120)\\=-0.8x+146$
The revenue: $n\times x=-0.8x^2+146$
