Contribution margin is the difference between the selling price and the variable cost. Contribution Margin Ratio is the ratio of contribution per unit divided by the selling price per unit.
So at $70 per unit of sales, the contribution per unit is
Contribution Margin = Selling Price - Variable Cost
Contribution Margin = $70 - ($70 × 40%)
Contribution Margin = $70 - $28
Contribution Margin = $42 per unit
Contribution Margin Ratio = Contribution Margin ÷ Sales Price per unit
Contribution Margin Ratio = $42 ÷ $70
Contribution Margin Ratio = 60%
Hence the Contribution Margin Ratio is 60%.