To find out how far Brian ran in 18 minutes, we first determine his running speed in kilometers per minute.
He ran 5 kilometers in 40 minutes:
\[ \text{Speed} = \frac{5 \text{ km}}{40 \text{ min}} = 0.125 \text{ km/min} \]
Next, we calculate how far he would run in 18 minutes:
\[ \text{Distance} = \text{Speed} \times \text{Time} = 0.125 \text{ km/min} \times 18 \text{ min} = 2.25 \text{ km} \]
So, Brian ran 2.25 kilometers in 18 minutes.