Calculate Average Speed: Converting 1½ km in 3 Minutes to Kilometers per Minute

To find out how many kilometers Joel ran on average each minute, we begin with these steps:

• Step 1: Convert the mixed number $1\frac{1}{2}$ km into an improper fraction. The mixed number represents 1 full unit plus half a unit, which is calculated as:

$1\frac{1}{2} = 1 + \frac{1}{2} = \frac{3}{2}$ km

• Step 2: Use the average formula to calculate the distance per minute:

$\text{Average distance per minute} = \frac{\frac{3}{2} \text{ km}}{3 \text{ minutes}}$

Simplifying this expression involves dividing fractions:

$\frac{\frac{3}{2}}{3} = \frac{3}{2} \times \frac{1}{3} = \frac{3}{2 \times 3} = \frac{3}{6} = \frac{1}{2}$ km per minute

Therefore, Joel traveled, on average, $\frac{1}{2}$ km per minute. This matches the provided correct answer, confirming choice 4 is correct.