You have a coupon for 10% off the sale price of a surfboard. which is the better buy? 40% off the regular price or 30% off the regular price and 10% off the sale price

40% off the regular price is better because:
Let's say the surfboard is $50. 40% off $50 means you are paying:
50 x 0.6 = $30. 
30% off $50 means you are paying:
50 x 0.7 = $35. 
Then 10% off $35 means you are paying:
$35 x 0.9 = $31.50. 
Therefore, it is cheaper to get 40% off the surfboard

Where are the functions?