Question 943904
First man built 1 wall in 10 days=1 wall/10 days=1/10 wall/day
Second man built 1 wall in 20 days=1 wall/20 days=1/20 wall/day
Together they build 1/10 wall/day +1/20 wall/day=(1/10)(2/2)wall/day + 1/20 wall/day=2/20 wall/day + 1/20 wall/day=3/20 wall/day
Together the men build 3/20 of the wall in 1 day.
To  build the whole wall {{{1 wall/(3/20 wall/day)=(20/3) days}}}=6 and 2/3 days
ANSWER Together it takes the men 6 and 2/3 days to build the wall.