Pore pressure at different point ( seepage in soil)

Homework Statement

I am not sure how to get Nd in this question ( circled part)

I am not sure the exact location of the point where the author want to find the pore pressure .

Can someone help ? This notes is confusing

Homework Equations

The Attempt at a Solution

I think the Nd can be only in whole number , so the 5.5 is definitely wrong .

 

Can someone point out the exact location of the point please ? Or i misunderstood something here ?