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 ?

83) The trigonometric value of Sin 30 is

84) Product of two negative integers is always