An increase in total revenue by the sale of an additional unit of the commodity is called Marginal Revenue.

Suppose the demand and supply curve of commodity X in a perfectly competitive market is given by:

$$\begin{gathered} q^D= 700 -p \\ {q}^S=500 +3p \mathrm{for} p\ge 15 \\ = 0 \mathrm{for} 0\le p<15 \end{gathered}$$

Assume that the market consists of identical firms. Identify the reason behind the market supply of commodity X being zero at any price less than Rs.$15$. What will be the equilibrium price for this commodity? At equilibrium, what quantity of X will be produced?

Suppose the demand and supply curves of salt are given by 

$$\begin{gathered} q^D=1.000-p \\ {q}^s=700 + 2p \end{gathered}$$

Now suppose that the price of an input used to produce salt has increased so that the new supply curve is 

$q^s=400+2p$

How does the equilibrium price and quantity change? Does the change conform to your expectation?