PLEASE HELP Write a system of equations for each of the descriptions below, and then solve the systems. Hint: you can graph the equations to find the solutions.

18. A golfer completed 18 holes with a total of 79 strokes. On each hole, he met par, which was either four or five each time. How many holes were played in four-strokes?

19. There are two numbers. The difference between them equals six, and one of the numbers is three times the other number. There are two pairs of numbers that satisfy these descriptions, What are they?

20. There is a number that is three-fifths the total of the other number. When four is added to one of the numbers their sum will be equal to the other number. There are two pairs of numbers that satisfy these descriptions. What are they?