Final answer:
Marta's mother is 45 years old.
Step-by-step explanation:
To find the age of Marta's mother, we need to determine what age is three times 15. Let's call Marta's mother's age M. According to the information given, M = 3 * 15. Multiplying 15 by 3, we get M = 45. Therefore, Marta's mother is 45 years old.