1 .

An aeroplane flies along the four sides of a square at a speed of 100, 200, 300 and 400 km/h, respectively. What is the average speed of the plane in its flight around the square?

[ A ]    196 km/h
[ B ]    200 km/h
[ C ]    250 km/h
[ D ]    None of the above
 Answer : Option D Explanation : Average speed = $Total\,\,\, Distance \over Total\,\,\, time$ = $x + x + x + x \over {x \over 100} + {x \over 200} + {x \over 300} + {x \over 400}$ = $4x \over {12x + 6x + 4x + 3x \over 1200}$ = $4x \times 1200 \over 25x$ = 192 km/hr