tolong nomor 30 beserta cara yaa makasih

r₁ - r₂ = 1/2 λ = 20 cm
r₁ - r₂ = 3/2 λ = 60 cm
r₁ - r₂ = 5/2 λ = 100 cm

panjang gelombang
½ λ = 40
λ = 40 cm = 0,4 m

frekuensi
f = v / λ
f = 340 / 0,4
f = 850 Hz ← jwb