To determine how far Brian ran in 18 minutes, we first need to find his running speed.
-
Calculate Brian's speed:
- Brian ran 5 kilometers in 40 minutes.
- His speed \( v \) in kilometers per minute is calculated as: \[ v = \frac{\text{distance}}{\text{time}} = \frac{5 \text{ km}}{40 \text{ minutes}} = \frac{5}{40} = \frac{1}{8} \text{ km/minute} \]
-
Calculate the distance covered in 18 minutes:
- The distance \( d \) Brian covered in 18 minutes can be calculated using the formula: \[ d = v \times \text{time} = \left(\frac{1}{8} \text{ km/minute}\right) \times 18 \text{ minutes} \]
- Multiplying these values gives: \[ d = \frac{1}{8} \times 18 = \frac{18}{8} = \frac{9}{4} = 2.25 \text{ km} \]
Thus, the distance Brian ran in 18 minutes is:
\[ \boxed{2.25} \]