P invested a certain amount of money on simple interest at the rate of interest 20% per annum for 3 years. Q invested some amount on the simple interest at the rate of interest 15% for 3 years. The sum of interests of P and Q after 3 years will be Rs. 6600 and the amount invested by P is Rs. 3000 less than the amount invested by Q. Find the total amount invested by P and Q together