What will be the ratio of the interests earned by identical sum of money invested at the same rate of simple interest for 6 years and for 15 years respectively?

2 : 5

Here, 

Everything is same except time, therefore

⇒ The ratio of the interests earned by identical sum of money invested at the same rate of simple interest for 6 years and for 15 years respectively = 6 : 15 = 2 : 5.