This is a great idea for students to really show what they know with numbers and solving equations.
I found the link on one of Resourceaholic's Gem lists. The post led me to Don Steward's Median blog. This is his activity. There are 3 posts: 'mobiles,' then 'mobile inequalities,' and finally 'mobile moments'.
I really like these activities! They are so adaptable to just about anything. Elementary students through high school students could work with adapted versions of these. Students do so much halving, doubling, sequencing, comparing, etc.
I can also see adapting this for fractions or decimals.
Maybe radians? Definitely integers...
This could even become an activity framework. It could be used for something like basic function notation, students evaluate to a number, then use that number to balance. (Almost as a self-check method)
Have a great weekend everyone!
UPDATE: There's also a website called SolveMe, which has these same types of problems, without numbers or expressions. It uses the same logic and reasoning skills, but with a bit friendlier interface. Here's the link.