Answer could be like this:
We know that
Cluster * Region * Routers = 720
We use the given formula to find minimum size of Routing table:
( Cluster-1 ) + ( Region-1 ) + Routers
Therefore, here also Cluster = 8, Region = 9 & Routers = 10
( 8-1 ) + ( 9-1 ) + 10 = 7 + 8 + 10 = 25.