National Australia Bank paid 20 times post-tax earnings for the mortgage distribution business of Challenger Mortgage Management.  NAB agreed to pay Challenger $385 million for Choice Aggregation Services, FAST and Plan in a deal announced last week. Challenger said it received "repeated approaches for the business" before agreeing to sell to NAB.The additional disclosure yesterday by Challenger (published as part of its June 2009 financial statements) implies annual earnings of about 17.5 million for the three aggregators. Challenger said a portfolio of $4 billion in mortgages to be sold to NAB (currently funded by a range of banks) contributed little profit.The most profitable, though declining, part of the business Challenger will keep for itself.Challenger Mortgage Management reported a higher yield from a shrinking mortgage book, the financial statements for the group show.EBIT increased to $136 million from $100 million in the year to June 2008. The return on net assets increased to 24 per cent from 21 per cent. The higher EBIT reflects the takeover by Challenger of Choice (bought in September 2007) and Plan (bought in September 2008)Challenger is retaining the back book (which is being sold to the life company). Interestingly, Challenger confirmed yesterday that the projected yield from the back book was lower than flagged only last week in announcing the sale of the distribution assets to NAB.The rate of decline in Challenger's mortgage book slowed over the second half of its financial year, to stand at $17.9 billion.