On a bicycle 5

Clara left the town square on her bicycle and rode toward the city at an average speed of 20 km/h. Johnny left the same square on his bicycle 15 minutes after Clara and rode in the same direction at an average speed of 30 km/h. How long did it take Johnny to catch up with Clara?

Final Answer:

t =  30 min

Step-by-step explanation:

$$\begin{gathered} v_1=20\text{km/h} \\ {v}_2=30\text{km/h} \\ \Delta t=15\text{min}\to \text{h}=15:60\text{h}=0.25\text{h} \\ {s}_1=s_2 \\ {v}_1\cdot (x+\Delta t)=v_2\cdot x \\ 20\cdot (x+0.25)=30\cdot x \\ 10x=5 \\ x=\frac{5}{10}=0.5 \\ x=\frac{1}{2}=0.5 \\ t=x\to \text{min}=x\cdot 60\text{min}=0.5\cdot 60\text{min}=30\text{min}=30\text{min} \end{gathered}$$

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