A boy and a girl together fill a cistern with water. The boy pours 4 liters of water every 3 minutes and girl pours 3 litres every 4 minutes. How much time will it take to fill 100 litres of water in the cistern?

Water filled by (boy + girl) in one minute,
$$\eqalign{ & \frac{4}{3} + \frac{3}{4} = \frac{{16 + 9}}{{12}} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \frac{{25}}{{12}}\,{\text{liter}} \cr} $$
Hence, time taken to fill 100 liter,
$${\text{ = 100}} \times \frac{{12}}{{25}} = 48\,{\text{minutes}}$$