In this particular case, the exercise is one that we've had since November 2011. This week we made a push to have videos created whenever an exercise doesn't have an accompanying explanation, which is why that video was created specifically for exercise.
Reviewing content of the exercises is a separate and important task which is ongoing – as you point out, the exercise itself has a few problems and we'll be sure to revisit its content in the coming months. Thanks for your useful criticism.
(And sorry for not responding yet directly on your blog or to you on Twitter – it was on my to-do list but as I opened Hacker News this morning, I felt the need to comment here, as response speed is key on a site like this.)
As I was looking through our content, I found we have a new exercise on comparing decimals! It was written just 10 days ago (and hasn't yet been added to the main site):
With questions like 0.3 __ 0.03 and 0.02 __ 0.1, I think you'll find that the exercise touches on exactly the misconceptions that you've seen. In addition, the hints use area models to explain the comparison, just as you suggest in your post.
Minor bug report, hints 3 and 5 (where the values are displayed as hundreths) overlap with the previous hints (where they are shown as combinations of tenths and hundredsths) for me on Firefox/Ubuntu, but look fine on Chromium/Ubuntu.
Reviewing content of the exercises is a separate and important task which is ongoing – as you point out, the exercise itself has a few problems and we'll be sure to revisit its content in the coming months. Thanks for your useful criticism.
(And sorry for not responding yet directly on your blog or to you on Twitter – it was on my to-do list but as I opened Hacker News this morning, I felt the need to comment here, as response speed is key on a site like this.)