6. According to six-tenths-factor rule, if the cost of a given unit at one capacity is known, then the cost of similar unit with '' times the capacity of the first unit is approximately equal to __________ times the cost of the initial unit.
8. An investment of Rs. 100 lakhs is to be made for construction of a plant, which will take two years to start production. The annual profit from the operation of the plant is Rs. 20 lakhs. What will be the pay back time?