What is the average speed for the entire trip?

Question

A train leaves on station 80 miles/hour for 3.5 hours and 70 miles/hour for 2 hours to reach it's destination. What was the average seed for the entire trip.

Answer 1

$$ \begin{aligned} \text{Average speed}&=\frac{\text{total distance traveled}}{\text{total time traveled}}\\[5pt] &=\frac{80\cdot 3.5+70\cdot 2}{3.5+2}\\[5pt] &=\frac{420}{5.5}\\[5pt] &=76.\overline{36} \end{aligned} $$

Answer 2

Find the total distance travelled (by summing up the distances of each "leg" at a different speed), then divide by the total time for the trip.