A train covers 's' distance at the constant speed `30 km h^(-1)` Then it covers the same distance in opposite direction at constant speed `45 km h^(-1),` then its average speed would be ......... km `h ^(-1).` (37.5, 36, 25)

Explanation

Average speed `= ("Total distance")("Total time") = (s +s)/(( s )/(30 ) + (s)/(45)) = (2s )/((s (45 + 30))/(30 xx 45 ))= (2s (30 xx 45))/( s (75)) = (2700)/(75) = 36 km h ^(-1)`