The owner of a milk store finds that, he can sell 980 litres of milk each week at 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 litres could he sell weekly at Rs 17/litre?
The relationship between selling price and demand is linear.
Assuming selling price per litre along X-axis and demand along Y-axis, we have two points (14, 980) and (16, 1220) in XY-plane.
We know that the equation of the line passing through the points (x1, y1) and (x2, y2) is given by 
∴ 
⇒ 
⇒ y – 980 = 120 (x – 14)
⇒ y = 120 (x – 14) + 980
When x = Rs 17/litre,
⇒ y = 120 (17 – 14) + 980
⇒ y = 120(3) + 980
⇒ y = 360 + 980 = 1340
Ans. The owner can sell 1340 litres weekly at Rs 17/litre.
17