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.?

80) In addition and subtraction of two integers, sign of the answer depends upon

81) An aero plane fly's along the four sides of a square at the speeds of 200, 400, 600 and 800 km/hr. Find the average speed of the plane around the field.

82) A motor car starts with the speed of 70 km/hr. with its speed increasing every two hours by 10 km/hr. In How many hours it will cover 345 km ?