83. To oscillate in four loops means n = 4 in Eq. 16-65 (treating both ends of the string as effectively “fixed”). Thus, λ = 2(0.90 m)/4 = 0.45 m. Therefore, the speed of the wave is v = fλ = 27 m/s. The mass-per-unit-length is µ = m/L = (0.044 kg)/(0.90 m) = 0.049 kg/m. Thus, using Eq. 16-26, we obtain the tension:
τ = v2 µ = (27)2(0.049) = 36 N.