Step-by-step explanation:
step 1. a triangle has 3 sides and the 3rd side cannot be longer than the other two sides added together
step 2. (and!) a triangle has 3 sides and the 3rd side must be at least as big as the difference between the 2 other sides
step 3. 15 - 10 < x < 10 + 15
step 4. 5 < x < 25
step 5. answer: 6.