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?

Amount of compost she bought = ' C ' cubic yards.
Each cubic yard of compost costs $25.
Amount she spent on compost  =  25C .

Amount of topsoil she bought = ' T ' cubic yards.
Each cubic yard of topsoil costs $15.
Amount she spent on topsoil =  15T .

Total volume she bought  =                 C + T       = 10 cubic yards
Total price for the whole truckload =  25C + 15T  =  $180

There's the system of equations you have to solve:

                                                    C +    T  =    10
                                                25C + 15T  =  180 .
Can you do that ?
Here.  Let me help.

Multiply the first equation
by 25, then write the second
one under it again:                  25C + 25T  =  250
                                                     25C + 15T  =  180 .

Subtractum:                                      10T  =  70

Divide each side by  10:                       T  =  7  cubic yards.

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