75) A discount series of 15% and 20 % is equal to a single discount of

76) On selling a calculator for Rs. 371, a man gains 6%. For how much should he sell it to gain 8%?

77) A and B can do a piece of work in twenty days. but A alone can do it in Thirty days. how long will B take to do it alone?

78) A tractor ploughs a field in fifteen days. Another tractor ploughs it is twenty days. In how many days could they plough the field together?

79) How many minutes would it take a policeman to overtake a motorist travelling at the rate of 60 km/hr. If the policeman is at a distance of one - quarter of a kilometer behind the motorist and the policeman is maintaining a speed of 65 km/hr.?