Dear Hans,

I found a command \determinenoflines by googling.
I made a MWE as following. It is working well.

But I have to write the paragraph twice for couning.
Is there a better way?

Thank you again.
Best regards,

Dalyoung

%%%%%%%
\def\StartHide{\doifmode{hideSolution}{\starteffect[hidden]}}   
\def\StopHide{\doifmode{hideSolution}{\stopeffect\blank[\the\noflines*line + \the\noflines*halfline]}}   

\enablemode[hideSolution]

\starttext

The first line.

\hrule
\StartHide
\samplefile{knuth}
\determinenoflines{\samplefile{knuth}}
\StopHide
\hrule

The second line.

\hrule

\StartHide
\samplefile{ward}
\determinenoflines{\samplefile{ward}}
\StopHide

\hrule

The last line.
\stoptext