A farmer has enough food to feed her 1000 cows for 50 days. After 10 days some of the cows run away and the food was now enough for the same period of 50 days as before. How many cows ran away?  Choose the correct answer:
A) 300 B) 200 C) 500 D) 400 E) 250

Rationale: 1000 - - - - 50 1000 - - - - 40 x - - - - 50 x * 50 = 1000 * 40 x = 800 1000 - - - - - - - 200 answer = b
Answer: B