log2 81 + log4 25

= log2 3^4 + log4 5^2
= 4 log2 3 + 2 log4 5
= 4 / log2 2 + 2 / log2 5
= 4 + 2/log2 5
= 4 + 2 / (log10 5 / log10 2)
= 4 + 2 * log10 2 / log10 5
= 4 + 2 / log10 5

Therefore, log2 81 + log4 25 = 4 + 2 / log10 5