Here 7 routers are given which are connected with a mesh topology, you have to find how many links they are connected through?

After:

and if you add one more router in the current mesh, how many links now you have to add?

use this formula:

no. of links= n(n-1)/2

where n = number of nodes

let us solve one as an example:

suppose we have 5 routers in a mesh and we have to find the number of links:

no. links= n(n-1)/2

= 5(5-1)/2 = 10 links