Assume that the number of apples is x and the number of oranges is y.
For the first given, we know that each apple costs $0.24 and each orange costs $0.8, therefore:
amount paid for apples = 0.24x and amount paid for oranges = 0.8y
we also know that the total amount spent is $12, therefore the first equation is as follows:
0.24x + 0.8y = 12
For the second given, we know that the total number of fruit bought is 20, therefore, the second equation is:
x + y = 20
You can easily graph these two functions and find a possible combination from the graph (the correct combination would be the intersection between the two lines).