The owner of a milk store finds that, he can sell `980` litres of milk each week a `Rs. 14//"litre"` and `1220` litres of milk each week at `Rs 16//"litre"`. Assuming a linear relationship between selling price and demand, how many lire could he sell weekly at `Rs 17//litres`?

Explanation

Let the selling price and demand be represented on `x`-axis and `y`-axis respectively, then two points `(14,980)` and `(16,1220)` be in the `xy`-plane. Equation of line passing through these points is
`y-980=(1220-980)/(16-14)(x-14)`
`=120(x-14)`
`implies =120x-1680`
`y=120x-700`
Now, at `x=17`
`y=120xx17-700`
`=2040-700`
`=1340`
Therefore, he can sell `1340` litres milk in a weak at `17rs` per litre.