Why do this problem?
is an exercise in logical thinking. It is a good
opportunity to insist on a precise explanation with
You could wrap up four identical boxes in gold paper and tell
the story, using a balance to compare the weights. Pose
the question and ask the children what should be done first, and
For each subsequent action, ensure that a reason is given. If
incorrect suggestions are made, do not correct them but carry out
the actions anyway. If children challenge them, ask for
When a satisfactory solution has been reached and everyone is
happy, pose the question about nine bars and give the children some
time to work on it in pairs. You might choose to give each pair a
large piece of paper to record their thinking, which they can then
share with the rest of the class.
You could then confirm a correct solution practically.
What would happen to the balance if the weights were eqaul?
What would happen if they weren't?
This version of an old favourite is made easier by specifying that
the fake weight is light. A suitable extension might be
to see what the minimum number of weighings would be if it wasn't
known whether the fake was light or heavy.
Done practically, the first part of this activity becomes
accessible for all.