Problem in applying $\log x \cdot\log y$ and $\log_a(a)$ in this question $\log_5 (10)\cdot\log_{10} (15)\cdot \log_{15} (20)\cdot\log_{20} (25)$
I thought I could apply $\log_a(a)$ but I am not able to understand how.

Hint
$$\log_{a}b=\frac{\log_{c}b}{\log_{c}a}$$
You can, for example, write
$$\log_{5}10=\frac{\log10}{\log5}$$
