There are innumerable things in life that can be arranged as a set of equations each with two variables. The only constraint for these equations to be created is that each of what is being considered as a variable uses the same resources as the other and the total amount of the resource utilized by two sets of both the variables is known.
For example, when I go to a super market I have a limited amount of money with me. Everything that I wish to purchase in the super market requires a particular amount of money. At a counter that has deals if I find the cost of 3 pounds of tomatoes and 2 pounds of potatoes equal to $8 and the cost of 4 pounds of tomatoes and 3 pounds of potatoes equal to $11, can I determine how much each pound of tomatoes cost and what is the cost of a pound of potatoes. This can be done by solving the following equations:
3T + 2P = 8
4T + 3P = 11
Solving the two, the cost of a pound of tomatoes is $2 and the cost of a pound of potatoes is $1.