Eighty kilograms (kg) of store material is to be transported to a location $10$ km away. Any number of couriers can be used to transport the material can be packed in any number of units of $10, 20$ or $40$ kg. Courier charges are Rs. $10$ per hour. Couriers travel at the speed of $10$ km/hr if they are not carrying any load, at $5$ km/hr if carrying $10$ kg, at $2$ km/hr if carrying $20$ kg and at $1$ km/hr if carrying $40$ kg. A courier cannot carry more than $40$ kg of load. The minimum cost at which $80$ kg of store material can be transported will be
- Rs. $180$
- Rs. $160$
- Rs. $140$
- Rs. $120$