STEP - BY - STEP EXPLANATION
What to find?
The price the retailer should marked them.
Given:
Cost per pair = $90
Sold at 10% discount
40% profit.
Selling price for 40% profit of the cost is;

Let S be the price the retailer mark them.


Divide both-side of the equation by 0.9

Hence, he need to mark them at the price of $140
ANSWER
$140