For the following velocity-time graph, the average speed for the...

As we know that, Distance = Area under velocity - Time graph $\therefore$ Total distance travelled in $80 \mathrm{~s}=$ Area of $\triangle O A B+$ Area of $\triangle B C D$$\begin{aligned}& =\left|\frac{1}{2}(A E \timesO B)\right|+\left|\frac{1}{2}(C F \timesB D)\right| \\ & =\left|\frac{1}{2} \times10 \times40\right|+\left|\frac{1}{2} \times-10 \times40\right| \\ & =200+200=400 \mathrm{~m} \end{aligned}$Total time taken, $t=80 \mathrm{~s}<img src="https://firebasestorage.googleapis.com/v0/b/quantrexacademy-app.firebasestorage.app/o/questions%2Ffigs%2Fdiagrams%2Fqx-self-cd14f7b763e9e349.png?alt=media&v=stem2" data-qx-orig-src="/assets/diagrams/qx-self-cd14f7b763e9e349.png" data-qx-storage-src="/assets/diagrams/qx-self-cd14f7b763e9e349.png">$ \\\begin{aligned} \text{ Now, the average speed } & =\frac{\text{ Total distance }}{\text{ Total time taken }} \\ & =\frac{400}{80}=5 \mathrm{~m} / \mathrm{s}\end{aligned}$$