Which tank will fill first?

Both 3 and 4

How?

• The water in tank 1 will get filled up to the connectivity of tank 1 and the tank 2 and then it will start flowing to tank 2.
• The water in tank 2 will get filled up to the connectivity of tank 2 and the tank 3 and then it will start flowing to tank 3.
• The water in tank 3 will get filled up to the connectivity of tank 2 and the tank 3 and then it will start flowing to tank 3.
• As the outlet of tank 3 and inlet of tank 4 are not at the same level, water will continue to fill tank 3 till the inlet of tank 4.
• Now both the tank 3 and tank 4 fill simultaneously until both fill together.