Motion against

From two places 36 km apart, a pedestrian and a cyclist will go straight opposite each other. How long will they meet if the speed of the pedestrian is 4 km/h and the cyclist 12 km/h?

Correct result:

t =  2.25 h

Solution:

$$\begin{gathered} v_1=4\text{ km/h } \\ {v}_2=12\text{ km/h } \\ s=36\text{ km } \\ v=v_1+v_2=4+12=16\text{ km/h } \\ t=s\mathrm{/}v=36\mathrm{/}16={\displaystyle \frac{9}{4}}={\displaystyle \frac{9}{4}}\text{ h}=2.25\text{ h} \end{gathered}$$

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