Use pie graphs to show the original tables split up into thirds of one girl two boys.
Then show two ways of combining two tables.
(>-) combined with (>-) is a table of six: 2 girls, 4 boys show all boys and girls moving from tables with 3 seats to the same table with 6 seats.
Now show a table with three seats, one occupied by a girl.
Show another table with same configuration.
One girl moves from one table to join the first table. 1/3 full + 1/3 full (at the same table) is 2/3 full.
Then show two ways of combining two tables.
(>-) combined with (>-) is a table of six: 2 girls, 4 boys show all boys and girls moving from tables with 3 seats to the same table with 6 seats.
Now show a table with three seats, one occupied by a girl. Show another table with same configuration. One girl moves from one table to join the first table. 1/3 full + 1/3 full (at the same table) is 2/3 full.