# Friday Fave for September 15

Summer is coming to an end in the Northern Hemisphere. Days are getting shorter; leaves are turning; 12-year-olds are working hard on percents…

That last one is why this week’s Friday Fave is the new percent of feature in the calculator.

From time to time, we at Desmos develop a feature or two because one or more partners needs us to. So it was recently with percent. A partner wanted a percent key.

The initial reaction was “There will be no percent key!”

But we dug deeper and soon got to the heart of the matterâ€”"Why do we object to a percent key?“

"Well…because they all behave unpredictablyâ€”different on every calculator and with undiscoverable rules. Like what does 55 + 10% even mean?!?”

And that’s when it hit us. Desmos could decide how the Desmos percent key would work, and that means Desmos can make it transparent!

So typing “%” now gets you “% of” and everything works exactly as it should.

55+10% is not well defined because we need to know what you want to find 10% of. The syntax makes that clear.

You can put a slider on your % of, and you can iterate to find percents of percents of values.

Transparent, functional percents native in the calculator. That’s this week’s Friday Fave.

Now while your mind is in the arena of percents, perhaps you’re also amenable to playing with some activities involving rational numbers more generally. Here are some suggestions: