(b) $2\left( \frac{v_{1}}{2\iota_{1}} \right) = \frac{v_{2}}{4\iota_{2}}$
$\therefore\frac{\sqrt{T/\mu}}{\iota_{1}} = \frac{320}{4\iota_{2}}\ $
($\mu$=mass per unit length of wire)
$Or\ \frac{\sqrt{50/\mu}}{0.5} = \frac{320}{4 \times 0.8}$
Solving we get $\mu$=0.02 kg/m=20 g/m
∴ Mass of string=20 g/m ×0.5 m=10 g