 Let $a$ and $b$ be two-digit positive integers. Find the greatest possible value of $a+b$, given that the greatest common factor of $a$ and $b$ is $6$.

[i]Proposed by Jacob Xu[/i]

[hide=Solution][i]Solution[/i]. $\boxed{186}$
We can write our two numbers as $6x$ and $6y$. Notice that $x$ and $y$ must be relatively prime. Since $6x$ and $6y$ are two digit numbers, we just need to check values of $x$ and $y$ from $2$ through $16$ such that $x$ and $y$ are relatively prime. We maximize the sum when $x = 15$ and $y = 16$, since consecutive numbers are always relatively prime. So the sum is $6 \cdot (15+16) = \boxed{186}$.[/hide]
1. Let \( a \) and \( b \) be two-digit positive integers such that the greatest common factor (GCF) of \( a \) and \( b \) is 6. This means we can write \( a \) and \( b \) in the form:
   \[
   a = 6x \quad \text{and} \quad b = 6y
   \]
   where \( x \) and \( y \) are integers.

2. Since \( a \) and \( b \) are two-digit numbers, we have:
   \[
   10 \leq 6x \leq 99 \quad \text{and} \quad 10 \leq 6y \leq 99
   \]
   Dividing these inequalities by 6, we get:
   \[
   \frac{10}{6} \leq x \leq \frac{99}{6} \quad \text{and} \quad \frac{10}{6} \leq y \leq \frac{99}{6}
   \]
   Simplifying, we find:
   \[
   1.6667 \leq x \leq 16.5 \quad \text{and} \quad 1.6667 \leq y \leq 16.5
   \]
   Since \( x \) and \( y \) must be integers, we have:
   \[
   2 \leq x \leq 16 \quad \text{and} \quad 2 \leq y \leq 16
   \]

3. Additionally, since the GCF of \( a \) and \( b \) is 6, \( x \) and \( y \) must be relatively prime (i.e., \(\gcd(x, y) = 1\)).

4. To maximize \( a + b = 6x + 6y = 6(x + y) \), we need to maximize \( x + y \) under the constraint that \( x \) and \( y \) are relatively prime and within the range \( 2 \leq x, y \leq 16 \).

5. The maximum sum \( x + y \) occurs when \( x \) and \( y \) are the largest possible values within the range and are relatively prime. The largest consecutive integers within the range are 15 and 16, and since consecutive integers are always relatively prime, we choose:
   \[
   x = 15 \quad \text{and} \quad y = 16
   \]

6. Therefore, the maximum value of \( a + b \) is:
   \[
   a + b = 6x + 6y = 6(15 + 16) = 6 \cdot 31 = 186
   \]

The final answer is \(\boxed{186}\).