Some persons can do a piece of work in 12 days. Two times the number of such persons will do half of that work in

Let x men can do the in 12 days and the required number of days be z

More men, Less days     [Indirect Proportion]

Less work, Less days     [Direct Proportion  ]

  $men2x:xwork1:\frac{1}{2} :: 12 : z$

$\therefore 2x\times 1\times z=x\times \frac{1}{2}\times 12$

  $\Rightarrow z=3$