How about an informal solution....
If all 9 fruits were oranges, the cost would be 9 times 50 cents, a total of $4.50.
But the total cost was $4.10, which is 40 cents less than $4.50.
Since each apple costs 10 cents less than each orange, and since $4.10 is 40 cents less than $4.50, the number of apples must be 40/10 = 4.
