The function for the cost of manufacturing lawnmowers is c = 0.008x^2-0.4x+75, where c is the total cost (in dollars) and x is the number of lawnmowers produced. how many units should be produced each day to yield a minimum cost

Jada has a meal in a restaurant she adds up the prices listed on the menu for everything they ordered and gets a subtotal of $42.00. when the check comes, it says they also need to pay $3.99 in sales tax. what percentage of the subtotal is the sales tax