Harvest time

Grandma and granddaughter Julia reap currants in 15 hours of working together. Julia herself would have currants ten days after 6 hours of work a day.
Find by what percentage the grandma's harvest time was shortened when the granddaughter was involved in the current harvesting.

Final Answer:

p =  25 %

Step-by-step explanation:

$$\begin{gathered} j=10\cdot 6=60\text{h} \\ 15/b+15/j=1 \\ 15/b+15/60=1 \\ 15/b=1-15/60=\frac{3}{4}=0.75 \\ b=\frac{15}{1-15/60}=20\text{h} \\ p=100\cdot \frac{b-15}{b}=100\cdot \frac{20-15}{20}=25\% \end{gathered}$$

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