Answer :

Time = distance/speed


Given, train covers a distance of 90 km at a uniform speed. Had the speed been 15 km / hour more, it would have taken 30 minutes less for the journey.


Let the usual speed be ‘a’.



90 ×(a + 15 –a) = (a2 + 15a)/2


a2 + 15a – 2700 = 0


a2 + 60a – 45a – 2700 = 0


a(a + 60) – 45(a + 60) = 0


(a + 60)(a – 45) = 0


a = 45 km/hr


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