I am trying to solve this question
Example: Alex and Sam also build tables. Together they make 10 tables in 12 days.
Alex working alone can make 10 in 30 days.
How long would it take Sam working alone to make 10 tables?
I calculated how many tables Alex can do in 1 day by doing 30/10 = 3.
I then did 12/3 = 4 to calculate how many Alex did in 12 days.
To find out how many tables Sam did in 12 days I did 12-4 = 8.
Now I can work out Sam's daily work rate by doing 12/8 = 1.5
If I do 1.5 * 10 then Sam will take 15 days to do 10 tables.
The website came to a different conclusion: So 10 tables would take Sam just 20 days.
Where has my logic gone astray?
Alex can do one table every three days. In the twelve days they are with Sam, they do $12/3=4$ tables. Together they all made $10$ tables, so Sam must have made the remaining $6$ tables in $12$ days, so their work rate is one table every two days. The website concludes correctly; $10$ tables times $2$ days equals $20$ days taken to make the $10$ tables.