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A tank holds 2500 litres of oil.
A small pump can add oil to the tank at a rate of x litres per minute.
A large pump can add oil to the tank at a rate of (x + 20) litres per minute.

(i) Write down an expression, in terms of x, for the number of minutes the small pump takes to fill the empty tank.
(ii) Write down an expression, in terms of x, for the number of minutes the large
pump takes to fill the empty tank.
(iii) It takes 15 minutes longer to fill the empty tank using the small pump than it does with the large pump.
Form an equation in x and show that it simplifies to
(iv) Find the length of time the large pump takes to fill the empty tank. Give your answer in minutes and seconds, correct to the nearest second.