A single petrol tank supplies the petrol to the whole city, while the petrol tank is fed by a single pipeline filling the tank with the stream of uniform volume. When the petrol tank is full and if 20,000 litres of petrol is used daily, the supply fails in 45 days. If 16,000 litres of petrol is used daily, it fails in 30 days. How much petrol can be used daily without the supply every failing? a) 28000 b) 47000 c) 24000 d) 42000