You can formulate your own equations by analyzing the given problem and its statements. You can do some illustrations so you can understand it better. Introduce some variables and the rest is algebra. For example:
An orange costs $2 while a banana costs $1.5. How many oranges and bananas do you have to buy such that the total cost would equal to $20. You bought a total of 12 fruits.
First, you have to introduce variables. Let 'x' be the number of oranges and 'y' be the number of bananas. One equation you can get from here is knowing the amount of total cost: 2x + 1.5y = 20. Then, the other equation would be knowing the amount of fruits: x+y=12. You have two unknowns and two equations. Hence, you can solve the problem. Solving them simultaneously, you would get that x=4 and y=8.
