What is the average oxidation state of copper in the superconductor $YB{a}_{2}C{u}_3{O}_7$?

Since the oxidation state of Y and Cu is +3 and +7 respectively.

$\stackrel{+3}{\mathrm{Y}}$ ${\stackrel{+4}{\mathrm{Ba}}}_2$$\stackrel{+7}{{\mathrm{Cu}}_3}{\stackrel{-14}{\mathrm{O}}}_7^{ }$

The ratio of Y and Cu is,

$\left(+\frac{7}{3}\right)$