First, you should find the unit price. So, divide .51 by 3, which equals 0.17. So, it costs $.17 for one sandwich roll. Next, multiply that number by 20, to get $3.40. So, it costs $3.40 for 20 rolls
I would love to help you but I don't have time because I really have to get this assignment done but if someone doesn't answer then later I will but I just want to let you know that your teacher sounds like a terrible person and I am really sorry she put you in that situation