Train A takes 1 hour more than train B to travel a distance of 720 km. Due to engine trouble, speed of train B falls by a third, so it takes 3 hours more than train A to complete the same journey. What is the speed of train A (in km/hr)?

Let the speeds of train A and train B be a and b. The first condition yields the equation (720 divided by b) minus (720 divided by a) equals 1. The second condition states that when train B's speed becomes 2b/3, its new time is 3 hours more than train A's time, yielding (1080 divided by b) minus (720 divided by a) equals 3. Subtracting the first equation from the second equation results in (360 divided by b) equals 2, giving b = 180 km/hr. Substituting b back into the first equation gives (720 divided by 180) minus (720 divided by a) equals 1, which solves to a = 80 km/hr.

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