worksheets are designed to be mental arithmetic activities.
are many ways in which these questions could be used. Possibly
the worksheets could be handed out to students (they are
ready to photocopy) but more likely only a selection of
questions will be used, perhaps from an overhead transparency.
Students could work on selected questions individually and
then discuss in groups, explaining their methods and why
the methods work.
see Teaching computation
in the calculator era for some general principles and
goals of teaching mental computation.