Question 1137567
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<pre>
Let x = # of the 18 p volumes, y = # of 12 p volumes.


From the condition, you have these 2 equations


         x +      y = 120       (1)    (counting volumes)

      18*x +   12*y = 1920      (2)   (counting money, in " p ")


From equation (1), express  y = 120 - x  and substitute it into equation (2), You will get


    18x + 12*(120-x) = 1920.


Express x and calculate answer


    x = {{{(1920 - 12*120)/(18 - 12)}}} = 80.


Then from equation (1),  y = 120 - 80 = 40.


<U>ANSWER</U>.  80 volumes at 18p and 40 volumes at 12p.


<U>CHECK</U>.   80*18 + 40*12 = 1920 p.   ! Correct !
</pre>

The problem solved using 2-equation setup and the Substitution method.