[pimlottc]

Nifty. Could use some explanation, as not everyone is familiar with the show Countdown (particularly outside the UK).

Even knowing the show, I was a bit confused how I was supposed to solve it using only one operation, until I realized it was a series of steps. And it took me a moment to figure out how to reuse the result of a previous step.

Once I had figured that all out, though, it was a pleasure to use. Well done!

[gdix]

There's a question mark icon help menu thing in the upper left.

[roey2009]

Don't your users skip the walkthrough/tutorial as well? I always assume they don't, and design the UI as intuitive as possible.