Mrs. smith runs a dessert parlor. their top selling items are waffles and milkshakes. she sells waffles for $5 each and milkshakes for $3 each. mrs. smith want to sell at least 60 waffles and milkshakes in all and wants to earn at least $210. if each waffle takes 7 minutes to make and each milkshake takes 3 minutes to make, how many waffles and milkshakes should she make to minimize the time spent making the two items?