Answer: 15
Explanation:
The ratio 3 : 4 represents the younger brother : older brother
3 + 4 = 7 
Since we want a total of 35, multiply each value in the ratio by 5:
 3 : 4 = 7
x5 x5  x5
15 : 20 = 35
So the younger brother had 15 oranges
and the older brother had 20 oranges