Maximum number of P – OH bonds in $H_4{P}_2{O}_8$ are ‘x’ ; the total number of P – O – P linkages present in of $P_4{O}_{10}$ are ‘y’ and orthophosphoric acid on gentle heat at ${250}^{0}C$ forms an acid with basicity ‘z’. The value of  $\frac{(x+y)}{z}$ is