Two buses

The first bus runs 15 minutes the second bus runs after 21 minutes. Together they both leave at 7:00 on Monday. When and what day will they meet?

Result

t = 8:45 hh:mm

Step-by-step explanation:

$$\begin{gathered} 15=3\cdot 5 \\ 21=3\cdot 7 \\ LCM(15,21)=3\cdot 5\cdot 7=105 \\ {t}_1=LCM(15,21)=105\text{ min } \\ {t}_2=t_1\to \text{ h}=t_1\mathrm{/}60\text{ h}=105\mathrm{/}60\text{ h}=1.75\text{ h } \\ t=7.00+t_2=7.00+1.75={\displaystyle \frac{35}{4}}=8.75=8:45\text{ hh}:\text{mm} \end{gathered}$$

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