Contribution is the difference between the Sales per unit and the variable cost per unit. When the selling price reduced to $6 per unit,
Contribution per unit = Sales per unit - Variable Cost per unit
Contribution per unit = $6 - ($6*0.6)
Contribution per unit = $2.4 per unit.
Now, the contribution margin ratio = Contribution per unit / Sales per unit
Contribution Margin Ratio = $2.4 / $6
Contribution Margin Ratio = 0.4 or 40%
Contribution Margin ratio when the sales price is $6 per unit is 40%.