Final Answer:
The age of Kelly is 20 years, thus the correct option is c.
Explanation:
The combined age of three friends is 50 years. Since the age of Kelly is twice the age of the youngest friend, it can be assumed that the youngest friend has an age of 10 years. Therefore, the age of Kelly is 20 years old (2 x 10).
In order to solve this problem, it is important to understand the relationship between the combined age of three friends and the age of the youngest. The combined age of three friends can be expressed as: Age of Youngest + Age of Middle + Age of Oldest = 50 years.
Since the age of Kelly is twice the age of the youngest, the age of the youngest can be expressed as: Age of Kelly/2. Therefore, the equation can be rewritten as: Age of Kelly/2 + Age of Middle + Age of Oldest = 50 years.
Substituting the expression for the age of the youngest into the equation, we get: Age of Kelly/2 + Age of Middle + Age of Kelly/2 = 50 years. This can be further simplified to: Age of Kelly + Age of Middle = 50 years.
Finally, solving for Age of Kelly, we get: Age of Kelly = 50 - Age of Middle. Since the age of the middle is not given, we can assume that it is equal to the age of the youngest, which is 10 years. Therefore, the age of Kelly is: Age of Kelly = 50 - 10 = 40 years.
From the above calculation, it can be concluded that the age of Kelly is 20 years old, which is twice the age of the youngest friend.