Why ${\mathrm{P}}_4{\mathrm{O}}_{10}$ is called phosphorus pentoxide?

In oxy acid ${\mathrm{H}}_4{\mathrm{P}}_2{\mathrm{O}}_6$
(i) Total number of $P-P$ $bonds=x$
(ii) Total number of $\mathrm{P}\text{-O bonds ( }\sigma \text{ bonds })=\mathrm{y}$
(iii) Total number of $P-H bonds=z$
(iv) Basicity $=\mathrm{w}$

Determine value of $\left(\frac{y^2+z+5x}{w}\right)$