Correct Answer - Option 2 : 5 km/hr
Given ∶
Time taken to complete his journey = 10 hrs
Speed increased from his usual speed = 10 km/hr faster
Time taken to finish the journey with increased speed = 3 hrs 20 min = 10/3 hrs
Formula Used ∶
Distance = Speed × Time
Calculation ∶
Let the slower speed be x and the faster speed be y
Then, y = x + 10
Distance covered with normal speed = Distance covered with faster speed.
x × Time taken = y × Time taken
⇒ x × 10 = ( x + 10) × (10/3)
⇒ 3x = x + 10
⇒ 2x = 10
⇒ x = 5 km/hr
∴ The slower speed is 5 km/hr.