The number of U.S. refinance applications jumped 6.4% in the latest week, according to the Mortgage Bankers Association, a result of mortgage interest rates falling to their lowest level in almost 17 months. The average rate on a 30-year fixed-rate conforming loan dropped to 4.23% last week from 4.33% the previous week. The chance for mortgage savings led more borrowers to apply for refinances and the refinance share of all mortgage activity rose to 42.2% from 39.7% the week before.