I think to alleviate your frustration you should just be honest about what you can/can't do up front, prior to the events.

Everyone is grateful for a volunteer, so if you just say you can't do any calculations but would love to meet and greet then they will know what to expect.

It still lessons the burden of work all around, and another body always helps with potty breaks and the like.

The alternative is for you to get frustrated and then not join in, and that turns into a lose/lose situation for you all.

And then you can concentrate on what you can do well, meeting and greeting and talking about the cause.