Answer:
6 days
Step-by-step explanation:
If the first bricklayer can build a wall in 15 days, he can build 1/15 of a wall in one day.
If the second bricklayer can build a wall in 10 days, he can build 1/10 of a wall in one day.
Therefore, if they work together, in one day they will complete
1/15 + 1/10 = 1/6
of the wall.
Since they can complete 1/6 of the wall in one day, they can complete 6/6 or one wall in 6 days.