The best way to solve this is to write it out part by part.
The first part is "the sum of 8 and 3," which is (8+3)
The second part is "...times a number," so we have n(8+3)
Lastly, "...is 23," means n(8+3) = 23, so here's the equation. In order to solve for n, divide both sides by (8+3) so 23/(8+3) which is 23/11. So the "number" is 23/11.