First pour a 3 litre of liquid in any other container instead of 3 and 5 litre containers. keep the five litre container above the other container so that the overflowing liquid goes into container placed below it. now in 5 litre container pour liquid with 3 litre container 2 times. now 3 +3- 6. but container is of 5 litre, 1 litre liquid will overflow and get into the other container. hence we got 4 litres. 3+1- 4