Rahul lent a certain amount of money to his friend at the rate of 6% per annum for 1^st 5 years, 8% per annum for next 7 years and 12% per annum for the period beyond 12 years, all at simple interest. If he received Rs. 8540 as total interest from his friend at the end of 15 years, then what was the amount of money Rahul lent to his friend?