To determine the equation representing the relationship between the tip \( t \) and the number of guests \( g \), we can analyze the given data:
- At 10 guests, the tip is $18: \( \frac{18}{10} = 1.8 \)
- At 20 guests, the tip is $36: \( \frac{36}{20} = 1.8 \)
- At 30 guests, the tip is $54: \( \frac{54}{30} = 1.8 \)
- At 40 guests, the tip is $72: \( \frac{72}{40} = 1.8 \)
- At 50 guests, the tip is $90: \( \frac{90}{50} = 1.8 \)
From this, we see that the tip per guest is consistently $1.80. This indicates that the total tip \( t \) varies directly with the number of guests \( g \). The relationship can be expressed as:
\[ t = 1.8g \]
Thus, the correct equation representing the relationship between the tip \( t \) and the number of guests \( g \) is:
\[ t = 1.8g \]
So the answer is \( t = 1.8g \).