Let Rahul income to be x.
Amount spend = (7/9)x
Save =x - (7/9)x = (2/9)x
ratio of it = spend / save = 7/9x/2/9x = 7:2
Now for Karan -
Let karan income to be y.
Spend money = 5/7y
Save money = y - 5/7y = 2/7y
Ratio =5/7y/2/7y = 5:2
Both save same.