Let's suppose the ratio of apples to oranges is 7:5.
To find the number of apples, what you do is multiply 48 by 7, then divide the product by 12. I got twelve from adding 7 and 5 together in the ratio. So you have 28 apples.
To find the number of oranges, you do the same thing, but with the number 5. So you have 20 oranges.
Therefore the ratio of apples to oranges is 28:20, which can of course by simplified into 7:5