To find the equation that best describes the information given:
- Let \( j \) be the number of pencils Josie has.
- According to the information, Josie has three times as many pencils as Marcy, who has 12 pencils.
This can be represented as:
\[ j = 3 \times 12 \]
or simplified to:
\[ j = 36. \]
However, the alternative description with the same relationship can be expressed as:
\[ 3j = 12, \]
indicating that if Marcy had one third of Josie's pencils, then Marcy would have 12, which matches the original condition.
Therefore, the correct equation is:
3j = 12.