A and B separately can build a wall in 12 and 16 days, respectively. If they work for 1 day alternatively, starting with A, in how many days will the wall be built?

$13\frac{2}{3}$ days

A = 12 days, B = 16 days,

⇒ They work on alternate days, starting with A,

 1st day = 4 units,

 2nd day = 3 units,

Every 2 days, 7 units of work is completed,

So, in 12 days, 42 units of work is completed,

and in 13 days, 46 units of work is completed,

Therefore, in \( {13 }_{ 3}^{2 } \) days, total 48 units of work is completed.