31数字题，文章中提到10 of the 31 wallets given to officers...说明共给了31只皮夹。

Over a six-week period, ABC news’ chief investigative correspondent Brian Ross and his team turned in 20 wallets or purses to 20 police officers chosen at random in Los Angeles. Varying amounts of cash were put in each, as well as numerous pieces of identification, with names, addresses and phone numbers.
     In fact, Ross used this technique back in the 1970s as a local reporter in Miami, when confidence in public officials at all levels was at an all-time low. The results of that wallet test did little to boost public confidences 10 of the 31 wallets given to officers in the Miami area were never recovered, and two of them were turned in but the cash was missing. A number of the officers were fired or took early retirement after that report.
     Almost 30 years later, police honesty and integrity are again being called into question, most recently in Los Angeles, where the police department is trying to recover from a serious corruption scandal in its Rampart Division.
     The Results: Of the 20 Los Angeles police officers who were given wallets and purses, every single one turned in the wallet and the money. Not a penny was missing from the wallets, which were given to officers of all races, throughout the city. "Police officers have only one legacy and that’s their integrity, their honesty," says Los Angeles Police Chief Bernard Parks. "Their word means a lot... and people believe in that badge and what it represents,"