Multiply the value by decimal number in latex?

I used the following construct quite often:

\includegraphics[width=\textwidth/2]{pic.png}


But when I try to adjust the size a small bit, like this:

\includegraphics[width=\textwidth*0.45]{pic.png}


I get

! Package calc Error: .' invalid at this point.


What am I doing wrong?

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The calc package allows only division by integers or by reals which are announced as \real: so

\includegraphics[width=\textwidth*\real{0.45}]{pic}


will do, but

\includegraphics[width=0.45\textwidth]{pic}


works as well, doesn't require calc and is faster.

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Thanks! I really should read a bit about TeX syntax - because this was not very obvious. –  Rogach Apr 23 '12 at 7:44

You can use

\includegraphics[width=0.45\textwidth]{pic.png}
`
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