[填空题]
A source emits sound of wavel6d8 p6zjk.8tu5z zrj0q vcrm 7engths 2.64 m and fp q, 93y8il3 0er yao)f9v fedaod3.j2.76 m in air. How many beats per s3ydp,q.yjrf l3 9 de8o3a a9i) ffev0oecond will be heard? (Assume T = 20 $^{\circ} $C)$\approx$ Hz(round to one decimal place)
