$$ Discount\ by\ May\ 15=20\% $$

Therefore,

$$ Fee\ paid\ for\ Grade\ 4\ child=\frac{80}{100}\times\$400=\$320 $$

and

$$ Fee\ paid\ for\ grade\ 6\ child=\frac{80}{100}\times\$500=\$400 $$

$$ \therefore Total\ Fee\ paid=\$ 400+\$320=\$720 $$