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13. Determine the height of a donkey.
  What is the height of a donkey, which is 8 feet less than a elephant has, and the height of a elephant is 3 times more than the height of a donkey?
 

Explanation:
x = y - 8; y = 3x. Therefore: x = 3x - 8; 2x = 8. Hence: x = 4.