Question

Helena uses a mixture of compost and topsoil in her garden. She purchased a total of 10 cubic yards of compost and topsoil for $180. If compost costs $25 per cubic yard and topsoil costs $15 per cubic yard, how many cubic yards of topsoil did she purchase?

Answer 1

let c = cubic yds of compost
and t = cubic yds of topsoil

it says the total amount of the compost and topsoil was 10 cubic yds

so c + t = 10

Answer 2

now, we can write another equation for the total cost

in general, the cost of (x) items at price (y) each, is x*y
applying this to our problem, x cubic yds of compost at $25 each ---> 25x
same logic with the topsoil ---> 15y

so 25x + 15y = 180

Answer 3

now, your system is:

c + t = 10
25x + 15y = 180

solve via substituion or elimination