Can you find 20 liters with 30 liter beaker and 40 liter beaker?

Yep you could. Fill the 30L beaker and empty it into the 40L beaker. The 40L beaker will now have room for another 10L of water. Fill the 30L beaker again and use it to fill the 40L beaker the rest of the way. Since only 10 more liters will fit into the 40L beaker, you're left with 20L in the 30L beaker.