franteraoka wrote:
Two bicycle wheels, A and B, start rotating at constant speeds at the same time., How many seconds after the wheels start rotatins will wheel B have made 6 more rotations than wheel A?
(1) The rate of rotation of wheel B is three times that of wheel A
(2) The two wheels average 40 revolutions per minute
(1) Let wheel A rotate 'x' times per unit time, so B rotates '3x' times per unit time. But without knowing 'x', we cannot answer the time it will take for B to make 6 more rotations than A. Not sufficient.
(2) Clearly this statement is not sufficient, as this doesnt tell us anything about rotations of A and B individually nor do we know which wheel makes more rotations.
Combining, let A makes x rotations per minute, and B makes 3x rotations per minute. Then: (x+3x)/2 = 40 or x = 20. So A makes 20 rotations per minute and B makes 60 per minute. In 60 seconds, B makes 40 more rotations than A. So to make 6 more rotations, B needs = 40/60 * 6 = 4 seconds. Sufficient.
Hence
C answer