Passage: Studies showed a single vertical stabilizer was insufficient at high angles of attack, and it was changed to twin vertical stabilizers, canted at 45°, resulting in a "relaxed longitudinal stability" design, which enhances maneuverability. Northrop was not yet confident in fly-by-wire controls and retained mechanically-signaled flight controls. The resulting aircraft, unveiled on 28 January 1971, advertised a maximum weight of 40,000 lb (18,144 kg) and maximum speed of Mach 2, but stirred little interest among foreign buyers.The first prototype (serial number 72-1569) was rolled out at Hawthorne on 4 April 1974; its first flight at Edwards AFB came on 9 June with Henry "Hank" Chouteau at the controls  flying for 61 minutes reaching an altitude of 18,000 feet and a maximum speed of 610 miles per hour. Chouteau afterwards remarked that, "When our designers said that in the YF-17 they were going to give the airplane back to the pilot, they meant it. It's a  fighter pilot's  fighter." The second YF-17 (s/n 72-1570) first flew on 21 August. Through 1974, the YF-17 competed against the General Dynamics YF-16. The two YF-17 prototypes flew 288 test flights, totaling 345.5 hours. The YF-17 attained a top speed of Mach 1.95, a peak load factor of 9.4 g, and a maximum altitude of over 50,000 ft (15,000 m). It could attain a sustained 34° angle of attack in level flight, and 63° in a climb at 50 kt (58 mph, 93 km/h).

Question: How many hours did the YF-17s fly?
345.5