A vessel of capacity 90 litres is fully filled with pure oil. Nine litres of oil is removed from the vessel and replaced with water. Nine litres of the solution thus formed is removed and replaced with water. Find the quantity of pure oil in the final oil solution?  Choose one
A) 83.9 litres  B) 54.9 litres  C) 37.9 litres  D) 72.9 litres  E) 27.9 litres
D   72.9 litres   Let the initial quantity of oil in vessel be T litres.   Let us say y litres of the mixture is taken out and replaced by water for n times, alternatively.   Quantity of oil finally in the vessel is then given by [(T - y)/T]n * T   For the given problem, T = 90, y = 9 and n = 2.   Hence, quantity of oil finally in the vessel   = [(90 - 9)/90]2 (90) = 72.9 litres.
<result>D</result>
A vessel of capacity 90 litres is fully filled with pure oil. Nine litres of oil is removed from the vessel and replaced with water. Nine litres of the solution thus formed is removed and replaced with water. Find the quantity of pure oil in the final oil solution?