One man's 1 day's work = $1/3$
One woman's 1 day's work = $1/4$
One boy's 1 day's work = $1/12$
(1 man + 1 woman) 1 day's work
= ${1/3}+{1/4}={7/12}$
Remaining work = $1$ - ${7/12} = {5/12}$
Now, $1/12$ work in done by = $1$ boy $5/12$ work is done by
= ${1 * 5/12}/{1/12}$
= $12/1$ x $5/12 = 5$
Hence, 5 boys shpuld assist one man and one woman to finish the work.