How to make an empty box

Surprised no one has mentioned this. Latex has feature specifically for questions and answers:

% allow for answer boxes:
documentclass[12pt]{exam}
printanswers

usepackage{amsmath}

begin{document}
begin{enumerate}

    % questions can be list items
    item (Problem 7 S 2.1)
    Evaluate the following expression using Euclid's algorithm (2.1.3). (textbf{Write each step down for credit})

    [gcd(98, 35)]

    % make an answer box
    begin{solutionorbox}[2in] 
    $98mod 35 = 28$ 
    $35mod 28 = 7$ 
    $28mod 7 = 0$ 
    $gcd(98, 35) = 7$
    end{solutionorbox}

    end{enumerate}
end{document} 

To add, if you need space for students to work on this printed out, you can use vspace.

% answer box with space
begin{solutionorbox}
vspace{10cm}
end{solutionorbox}

To make empty boxes that align nicely with the text, you can combine fbox with phantom in this way:

fbox{phantom{This is my answer}}

You set the width and height by the width and height of the text in phantom. The resulting box will align perfectly with a line of text, contrarily to the box created with framebox .

For the record, and probably not the "best" solution, here is another technique. I used it before discovering the above accepted answer. It can be useful if your document already uses the "listings" package. If not, you probably should rely on above solution.

It produces an empty framed box whose height is exactly the number of empty lines you put inside.

MWE:

documentclass{article}    
usepackage{listings} 
lstset{
    frame=single,
    showlines=true
}

begin{document}
text before
begin{lstlisting}




end{lstlisting}
text after
end{document}