Courier delivery cars

The courier company delivers the ordered goods through the e-shop. If the delivery were made by 2 cars, it would be completed in 6 hours. However, after 4 hours, the first car's driver stopped delivering, and the second car continued to deliver for another 6 hours. How many days would the delivery be completed if only the second car had the goods?

Final Answer:

b =  18 h

Step-by-step explanation:

$$\begin{gathered} 6/a+6/b=1 \\ 4/a+4/b+6/b=1 \\ 6/a=1-6/b \\ a=6/(1-6/b) \\ 4/(6/(1-6/b))+4/b+6/b=1 \\ 4\cdot (1-6/b)/6+10/b=1 \\ 4\cdot (b-6)/6+10=b \\ 2b=36 \\ b=\frac{36}{2}=18=18\text{h} \\ b=18 \\ \text{ Verifying Solution: } \\ a=6/(1-6/b)=6/(1-6/18)=9\text{h} \\ {t}_1=6/a+6/b=6/9+6/18=1 \\ {t}_2=4\cdot (1/a+1/b)+6/b=4\cdot (1/9+1/18)+6/18=1 \end{gathered}$$

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