A Martian bat flies at 60 yards per second from its nest to a dry lake. When it arrives there, it immediately continues at 45 yards per second to a nearby cave. If the distance between the lake and the cave is half the distance between the nest and the lake, what is the average speed, in yards per second, of the bat during the whole flight? Choose the correct answer:	A) 36	B) 40	C) 42	D) 54	E) 65

Rationale: distance from nest to lake = d , speed = 60 , time = d / 60 distance from lake to cave = d / 2 , speed = 45 , time = d / 90 average speed = ( d + d / 2 ) / ( d / 60 + d / 90 ) = ( 3 / 2 ) / ( 1 / 36 ) = 54 answer : d
Answer: D