Manoj and his wife Swathi fill a tank with water fetched from a well. Manoj fetches 5 litres of water in 4 minutes whereas Swathi fetches 4 litres of water in 5 minutes. Find the time taken by them together to fetch 205 litres of water.

100 minutes

Every 4 min = 5 L water is added and Every 5 min = 4 L water is added,

4 min = 5 L       and 5 min = 4 L

8 min = 5 L             10 min = 4 L

12 min = 5 L           15 min = 4 L

16 min = 5 L            20 min = 4 L

20 min = 5 L            20 min = 16 L

20 min = 25 L

Therefore, in every 20 minutes, (25 + 16) = 41 L of water is added,

Hence in 100 minutes , 205 litres of water is added.