A bus left 60 minutes later than the scheduled time but to reach its destination 300 km away in time, it had to increase its usual speed by 10 km/hr. What is the usual speed of the bus?
Show answer & explanation
Correct answer: B
Explanation
The correct answer is option 2 i.e. 50 km/hr.
Let the speed of the bus be x.
⇒ 300/x - 300/(x + 10) = 1
⇒ (300(x + 10) - 300x)/x(x + 10) = 1
⇒ (300x + 3000 - 300x)/x(x + 10) = 1
⇒ 3000/x(x + 10) = 1
⇒ 3000 = x(x + 10)
⇒ x2 + 10x - 3000 = 0
⇒ x2 + 60x - 50x - 3000 = 0
⇒ x(x + 60) - 50(x + 60) = 0
⇒ (x - 50)(x + 60) = 0
⇒ x = 50, -60
The speed of the bus is 50 km/hr