Not breaking the start of an environment for solution of exercises

Question

Let suppose that I have an environment \begin{sol} ... \end{sol} for writing solutions of exercises in a test. For practical reasons, I would like that two or more solutions can be put in the same page only if their whole contents are only in this page.

For example, if the solutions of exercises 1 and 2 are only in the page, they can be put together, but if not, the solution 1 must be alone, and then the solution 2 must starts at a new page.

The main problem is that the sizes of the solutions are not fixed, and they can be smaller or not that one page. It can have one small solution followed by another small one, or not.

The number of solutions is not fixed neither. Indeed, in reality I need less than 10 solutions.

Answer 1

Having read the "don't put answers in comments" instructions, here is my "\filbreak" answer as an answer.

\documentclass[12pt,a5paper]{article}
\usepackage[margin=20mm]{geometry}
\usepackage{lipsum}

\newenvironment{sol}{\paragraph{Solution:}}{\filbreak\bigbreak} 

\begin{document}
\begin{sol} \lipsum[3] \end{sol}
\begin{sol} \lipsum[3] \end{sol}
\begin{sol} \lipsum[3-10] \end{sol}
\begin{sol} Very short one \end{sol}
\begin{sol} Very short one \end{sol}
\begin{sol} \lipsum[3] \end{sol}
\begin{sol} Very short one \end{sol}
\end{document}

Answer 2

Here is an approach that will allow solutions longer than a page. We typeset them once, and measure their height, if they will not fit on the current page then we issue a \clearpage first.

\documentclass{article}

\usepackage{etoolbox,environ}
\newsavebox{\mytmpbox}

\newcounter{solution}

\NewEnviron{sol}{\setbox\mytmpbox\vbox{\bigskip
\stepcounter{solution}\noindent{\textbf{Solution~\thesolution.}} \BODY\par}%
\ifdimgreater{\ht\mytmpbox}{\pagegoal-\pagetotal}{\clearpage}{}%
\unvbox\mytmpbox\bigbreak
}

\usepackage{lipsum}

\begin{document}

\begin{sol}
  \lipsum[1]
\end{sol}

\begin{sol}
  \lipsum[2]
\end{sol}

\begin{sol}
  \lipsum[3-10]
\end{sol}

\begin{sol}
  \lipsum[11]
  \begin{equation}
    x = y
  \end{equation}
  \lipsum[12]
\end{sol}
\end{document}

This places solutions 1 and 2 on page 1, solution 3 on the top of page 2 spreading to page 3 and solution 4 in the remaining space on page 3.

Page 1:

Pages 2 and 3:

Answer 3

In the follwoing solution I "designed" a very simple look for the solution environment. Feel free to change it/customize it to your needs. However, the style is not important for the solution, which consists in wrapping the solution body into a \vbox. This will alredy do the job.

In the example "Solution 1" and "Solution 2" can stay on one page, but "Solution 3" does not fit on the same page, hence it breaks to the next one.

The Code

\documentclass{article}
\usepackage{environ}
\usepackage{lipsum}

\newcounter{sol}
  \providecommand{\solname}{Solution}
\NewEnviron{sol}{%
  \parindent=0em
  \par
  \stepcounter{sol}
    \vbox{%
    \bgroup\itshape \solname\space\thesol.\egroup
    \par
    \BODY
    }
  \par\bigskip
}

\begin{document}
\begin{sol}
  \lipsum[1]
\end{sol}
\begin{sol}
  \lipsum[2]
\end{sol}
\begin{sol}
  \lipsum[3-6]
\end{sol}
\end{document}

Output