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I think the key to getting this question quickly is to recognize that both scenarios where the cars broke down are identical except for what happened between the 18 and 30 km mark. The first car travels this distance of 12 kms at a speed of (4/5) s while the second car travels this distance at the full speed of s. And this difference in speed creates a difference in time of 45 - 36 = 9 mins.

From there we have:
Time difference = (distance / speed) of car 1 - (distance / speed) of car 2 during those 12 kms.

9 mins = (9/60) hrs = (12 km / (4s/5)) - (12 km / s) ... Here s is speed and not seconds.

9/60 = (12x5/4s) - (12/s)

9/60 = 15/s - 12/s

9/60 = 3/s

s = 3 x 60 /9 = 20 km/hr.

C is the only option with this so I stopped there.
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