A jogger runs 3.0 km in the first 10 minutes, rests for 5 minutes, then runs the remaining 1500 m in 5 minutes. Find the average speed over the whole outing.

Step-by-step solution

1. 1

   Put both distances in the same unit. $3.0\text{ km}=3000\text{ m}$, so the total path is $s=3000+1500=4500\text{ m}$

2. 2

   Include the rest in the total time. Average speed is defined over the elapsed time, so the 5-minute break is part of the denominator: $t=10+5+5=20\text{ min}$

3. 3

   Divide. $v_{tb}=\frac{4500}{20}=225\text{ m/min}$

4. 4

   See where the distractors come from. Dropping the rest gives $4500/15=300\text{ m/min}$ (option A), and using only the second run gives $1500/20=75\text{ m/min}$ (option C). Both are the classic errors this question tests.

5. 5

   State the answer. The average speed is $225\text{ m/min}$, option B, which is $3.75\text{ m/s}$ in SI units.