In a family, the income ratio of three working members - A, B and C is 4 : 6 : 5. The ratio of their expenses is 3 : 5 : 4. Each of them saves Rs. 12,000 per month. If D, with a monthly income equal to the combined income of A and B, joins the family and his expenditure to income ratio is equal to the family's total expenditure to total income ratio before he joined, how much does D save per month?