If A can do $\frac{1}{5}$ th part of the work in 12 days, B can do $60\%$ of the work in 30 days and C can do $\frac{1}{3}$rd of the work in 15 days, then the work completes fast is

$A-{\frac{1}{5}}^{th}in-12\text{\hspace{0.17em}days\hspace{0.17em}\hspace{0.17em}remainig}=\frac{4}{5}\}\to 5$

$B-{\frac{3}{5}}^{th}in-30\text{\hspace{0.17em}days\hspace{0.17em}remaining}=\frac{2}{5}\}\to 5$

$C-{\frac{1}{3}}^{rd}in-15\text{\hspace{0.17em}days\hspace{0.17em}\hspace{0.17em}remainig}=\frac{2}{3}\}\to 3$

A takes to do work $=12\times 5=60\text{\hspace{0.17em}days}$

B takes to do work $=30\times \frac{5}{3}=50\text{\hspace{0.17em}days}$

C takes to do work $=15\times 3=45\text{\hspace{0.17em}days}$

$\therefore$ C complete fastly.