Brian will have 15 stickers left after 5 days.
To find this answer, you first multiply 5 by 5 (number of stickers given to his friends multiplied by the number of days he gives the stickers out). This will give you 25 stickers that are given out after 5 days.
Next, subtract 25 from 40, to find the number of stickers left over. Thus making 15 stickers, the answer.
I hope this helps!