Linear Equation type of question (fuel mixture)

here is the question:

Premium gasoline sells for 78.9cents/Liters. Regular gas sells for 71.9 cents/ litres. To boost sales, a middle octane gasoline is formed by mixing premium and regular. If 1000 L of this middle octane gas is produced, and is sold at 73.9 cents/Litres, then how much of each type of gasoline can you assume was used in the mixture?

Answer : 600 L premium and 400 L regular.........i got the answer from the back of the buk.....but i m not able to solve this question and get this answer

help plz