Short class periods, so just try this:
Open this (click run) http://phet.colorado.edu/en/simulation/balancing-act
Complete levels 1-4 on the GAME until you know what you are are doing,
then work with your table to find the unknown masses in the BALANCE LAB
Open this (click run) http://phet.colorado.edu/en/simulation/balancing-act
Complete levels 1-4 on the GAME until you know what you are are doing,
then work with your table to find the unknown masses in the BALANCE LAB