WASHINGTON (AP) — The average 30-year U.S. mortgage rate remained at a 52-week low of 4.10 this week.Mortgage company Freddie Mac also said Thursday that the average for a 15-year mortgage, a popular choice for people who are refinancing, rose to 3.25 percent from 3.23 percent.At its 52-week low of 4.10 percent, the rate on a 30-year mortgage is down from 4.53 percent at the start of the year. Rates have fallen even though the Federal Reserve has been trimming its monthly bond purchases, which are intended to keep long-term borrowing rates low. The purchases are set to end in October.The low rates appear to have boosted U.S. home sales.