Sidney kept track of the trick-or-treaters who came to her door and found that 1/8 were dressed as ghosts and 5/12 were dressed as witches. What fraction of the trick-or-treaters were dressed as either ghosts or witches?

To find the fraction of trick-or-treaters who were dressed as either ghosts or witches, we need to add the fractions of those dressed as ghosts and those dressed as witches.

The fractions given are:

• Ghosts: \( \frac{1}{8} \)
• Witches: \( \frac{5}{12} \)

To add these fractions, we need a common denominator. The denominators are 8 and 12. The least common multiple (LCM) of 8 and 12 is 24.

Next, we convert each fraction to have the denominator of 24.

1. Convert \( \frac{1}{8} \): \[ \frac{1}{8} = \frac{1 \times 3}{8 \times 3} = \frac{3}{24} \]

2. Convert \( \frac{5}{12} \): \[ \frac{5}{12} = \frac{5 \times 2}{12 \times 2} = \frac{10}{24} \]

Now we can add the two fractions: \[ \frac{3}{24} + \frac{10}{24} = \frac{3 + 10}{24} = \frac{13}{24} \]

Therefore, the fraction of the trick-or-treaters who were dressed as either ghosts or witches is: \[ \boxed{\frac{13}{24}} \]