__Example 1__**:**

A milkman can sell 650 liters of milk at Dollars 3.15 per liter and 800 liters of milk at Dollars 3.00 per liter. Assuming the graph of the sale price and the milk sold to be a straight line, find the number of liters of milk that the milkman can sell at Dollars 2.50 per liter.

__Solution__**:**

Let be the number of liters of sold milk and be the per liter price. The given information can be written in the form of points and . Since the graph of sale price and the milk sold is a straight line, so we find the equation of straight line through the points and as follows:

Using the two points form of equation of a straight line ** **

In order to fine the number of liters of milk that the milkman can sell at Dollars, put in the above equation, we get

This result shows that the milkman can sell 1301 liters of milk at Dollars 2.50 per liter.

__Example 2__**:**

Find the equation of straight line passing through the points and .

Consider the points and , now using these points in two point form of equation of straight line, we get

Which is the required equation of straight line.