The distance between towns A and B is 300 km. One train departs from town A and another train departs from town B, both leaving at the same moment of time and heading towards each other. We know that one of them is 10 km/hr faster than the other. Find the speeds of both trains if 2 hours after their departure the distance between them is 40 km.
Choose the correct choice:
A) 22 B) 27 C) 236 D) 90 E) 81

Rationale: let the speed of the slower train be xx km / hr . then the speed of the faster train is ( x + 10 ) ( x + 10 ) km / hr . in 2 hours they cover 2 x 2 x km and 2 ( x + 10 ) 2 ( x + 10 ) km , respectively . therefore if they did n ' t meet yet , the whole distance from a to b is 2 x + 2 ( x + 10 ) + 40 = 4 x + 602 x + 2 ( x + 10 ) + 40 = 4 x + 60 km . however , if they already met and continued to move , the distance would be 2 x + 2 ( x + 10 ) − 40 = 4 x − 202 x + 2 ( x + 10 ) − 40 = 4 x − 20 km . so we get the following equations : 4 x + 60 = 3004 x + 60 = 300 4 x = 2404 x = 240 x = 60 x = 60 or 4 x − 20 = 3004 x − 20 = 300 4 x = 3204 x = 320 x = 80 x = 80 hence the speed of the slower train is 60 km / hr or 80 km / hr and the speed of the faster train is 70 km / hr or 90 km / hr . answer : d
Answer: D