Why do this problem?
Whether this problem
is done with a balance or with pencil and paper, there is more to it than just doing addition. Although addition and subtraction are involved, you are likely to find the children using all sorts of language associated with these two operations, and one of the main points here is to establish a balance or equivalence as well
as "equals". Some pupils are not so readily used to coming across situations like $3 + 5 = 6 + 2$, for example. You could use this activity to introduce some algebraic ideas.
You may encourage the class to articulate what they are trying to find in a general sense. For example "$7$ balances two other numbers", or "$7$ = 'something' add 'something'"or "$7$ = ? $+$ ?". This will help them to get the idea that they are finding different numbers which fit this criterion - not just one answer.
How might you start?
How might you record your thinking?
How do you know you've got all the possibilities?
As the question suggests, some learners might like to look at a balance involving other fractions.
Children might like to try Getting the Balance