Bunuel
A and B are two diametrically opposite points on a circular road whose circumference is 6 km long. A cyclist started from point A and made two circles. He made the first circle with a certain uniform speed and then decreased his speed by 3 km/hr. The interval between his two circles through point B is 50 minutes. What is the speed at which the cyclist made the first circle?
A. 6 km/r
B. 7 km/r
C. 8 km/r
D. 9 km/r
E. 10 km/r
Solution:The distance between A and B around the road is 3 km since the entire circumference of the circular road is 6 km.
We can let r = the speed of the cyclist in completing the first circle. Notice that the distance traveled between the first time he passes through point B and the second time he passes it is the circumference of the road, or 6 km. However, the first 3 km of this 6 km is the 2nd half of the first circle in which his speed is r km/hr and the last 3 km is the 1st half of the second circle, in which his speed is (r - 3) km/hr. Therefore, we can create the equation:
3/r + 3/(r - 3) = 5/6
Multiplying the equation by 6r(r - 3), we have:
18(r - 3) + 18r = 5r(r - 3)
18r - 54 + 18r = 5r^2 - 15r
5r^2 - 51r + 54 = 0
(5r - 6)(r - 9) = 0
r = 6/5 or r = 9
Although both values of r are positive, r can’t be 6/5, otherwise, r - 3 would be negative. Therefore, r = 9.
Answer: D