A charade, how to do that?

The charade how to measure 20ml the lower quantity is 60ml.

First you measure 80ml pour that quantity in another cup, and after measure of from the 80ml just 60ml and what rest in the cup that isn't graduate is the 20ml. Rute Bezerra de Menezes Gondim

Ps.: Guide said that people doesn't see solution if the solution is in the other cup that isn't the graduated one. So is simple but head doesn't work like that see the solution right in the empty.