( Average total cost) = ( average fixed cost) + ( average vairable cost)
$\;{\;\text "totalcost"\;}/{\;\text "quantity"\;}$ + $\;{\;\text "(total variable cost)"\;}/{\;\text "quantity"\;}$
$\frac{$600}{100}$ + $\frac{$400}{100}$
= \$6 + \$4 = \$10
