Question 1202301
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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