SOLUTION: The mass of a substance varies directly as the volume of the substance. If the mass of 7 liters of the substance is 42 kilograms, what will be the volume in liters of 63 kilograms

You can calculate the density (mass per unit volume) easily enough:

42kg/7l = 6kg/l



Now you can use the density to compute volume of a given mass, by dividing:

  V = 63kg / (6kg/l) =  10.5l 





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For this problem,  a proportion would also work:



  7/42 = x/63  -->  x = 63*7/42 = 10.5