Concept: The question tests on the basics of Rates(Time Speed)
Solution:
Let the correct time taken by Dimpy = t hours
By the law of proportions, when distance is constant s1 * t1 = s2 * t2
s1 = 10 km/h and t1 = (t + 7/60) (Converting minutes to hours)
s2 = 12 km/h and t2 = (t - 8/60)
Therefore 10 * (t + 7/60) = 12 * (t - 8/60)
10 t + (70/60) = 12t - (96/60)
166/60 = 2t
t = 83/60
Putting t in any of the equations will give us the distance.
10 * (83/60 + 7/60) = 10 * 90/60 = 15 km
Option A