A level balance has some of one kind of object on one side and some of another kind on the other. The mass of one kind is given, and the student works out the mass of the other.
Level means both sides weigh the same: the idea behind solving equations, met here before any letters appear. Every answer is a whole number.
Numbers&Me is free. Sign up to practise more, or set it as homework in under a minute, and it marks itself.