Question

the shampoo you like to use is on sale. but you only get the lower price if you buy 3 bottles. you want to know how much youll save. write a formula that will help you determine how much you will save by buying 3 bottles on sale
let s= sale price per bottle, r= regular price per bottle, and c= cost savings.

Answer 1

The best way to go about this is to make up a scenario:

Suppose shampoo costs $5 per bottle, which would amount to $15 if you purchased thee bottles. However, you can buy 3 bottles for $12

To find the sales price, you would divide $12/3 = $4. So you're basically saving $3

so in essence
r = $5
s = $4
c = $3 ($1 per bottle)

As for a formula:

It will in general be

Regular Price - Sales Price = Cost Savings...so

r - s = c

In this case $5 - $4 = $1 meaning you'll save $1 per bottle.

Answer 2

r x 3 = rc

s x 3 = sc

rc - sc = c