A bus covered a distance of 250 km, partly at an average speed of 40 kmph and partly at 60 kmph. If the total time taken is 6.5 hours, then the distance covered at 40 kmph is
Choose the correct choice from the following
A) 130 km  B) 120 km  C) 100 km  D) 280 km  E) none of these

Rationale: "let the partial distance covered at 40 kmph be x let the another partial distance covered at 60 kmph be ( 250 - x ) thus , x / 40 - ( 250 - x ) / 60 = 6.5 or , x / 40 + ( 250 - x ) / 60 = 6.5 or , ( 3 x + 500 - 2 x ) / / 120 = 6.5 or 500 + x = 780 x = 280 answer : d"
Answer: D