Answer:
23
Explanation:
if you have 8 apples in a box, and then you put 15 additional apples into the box - how many apples are in the box ?
you see that is the same problem as the symbolized scenario with 2 lines being connected.
you have a string (e.g. for a kite) of 8 meters. and then you tie another string of 15 meters to it - how long is the new combined string ?
you really need help with that ?
8 + 15 = 23