Let \( x \) be the number of tickets sold last week. According to the problem, 300 tickets sold this week represent 120% of the tickets sold last week. From this, we can write the equation:
\[ 300 = 1.2x \]
To find \( x \), we can solve for \( x \) by dividing both sides of the equation by 1.2:
\[ x = \frac{300}{1.2} \]
Calculating the right side:
\[ x = 250 \]
Thus, the number of tickets sold last week is \( \boxed{250} \).