First you find the proportions used
78.94*x+71.94*(1-x)=73.94
78.94*x+71.94-71.94*x=73.94
78.94x-71.94x=73.94-71.94
7x=2
x=2/7
x=0.2857
Multiply this proportion with the overall amount (1000 litres)
and you get
285.714 liters of premium
and 1000-285.714 of regular.