Reading records

Lucy, Tereza, and Petra decided to compile the compulsory reading records together. It would take 30 days for Lucia alone, 36 for Tereza, and 45 for Petra. How long will it take to make records if they will work together?

Final Answer:

t =  12 d

Step-by-step explanation:

$$\begin{gathered} h=\frac{1}{30}+\frac{1}{36}+\frac{1}{45}=\frac{6}{180}+\frac{5}{180}+\frac{4}{180}=\frac{6+5+4}{180}=\frac{15}{180}=\frac{1}{12}\doteq 0.0833 \\ t\cdot h=1 \\ t=1/h=1/0.0833=12\text{d} \end{gathered}$$

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