55) The inlet valve of a water tank of capacity 18 m3 is set at a flow rate of 0.3 litres /sec. The time required to fill the tank is

56) The presence of lead in water can be detected by using

57) One unit of purified water is equivalent to

58) The minimum diameter of working chamber of a circular manhole should be

59) The mass of 1 cubic meter of cement concrete is