This is why I taught my kids to label units. ( 1 girl / 3 students at table L + 1 girl / 3 students at table R ) can't be added, because the denominators are d

This is why I taught my kids to label units. ( 1 girl / 3 students at table L + 1 girl / 3 students at table R ) can't be added, because the denominators are different. To fix that, you can multiply by the factors ( 3 students at table L / 6 students at both tables ) and ( 3 students at table R / 6 students at both tables ) so you get ( 1 girl / 6 students at both tables + 1 girl / 6 students at both tables ) = ( 2 girls / 6 students at both tables ). To hammer it home, you point to the table that has 2 or 4 students at it, and ask someone how to add the proportion of girls at that table to the two already under consideration.