Mohan's 1 day's work = $1/10$
Sohan's 1 day's work = $1/12$
Mohan and Sohan's 10 days work (working alternately by)
= $5 × 1/10 + 5 × 1/12 = 11/12$
Remaining work = $1 - 11/12 = 1/12$
Now,$1/10$ work is done by Mohan in = 1 day
$1/12$ work is done by Mohan in = $1/{1/10}$ × $1/12$ = $10/12$ = $5/6$
Hence, number of days to finish the work = 10 + $5/6$ = $10{5/6}$