Simplicity was a key for Redmond’s Aidan Hughes when he was programming the application that is currently ranked No. 3 for the Kindle Fire.

That, and his 13-year-old daughter.

In December of 2011, as Isabel Hughes worked on her math homework — fractions — she would check her answers with her dad. The two found it to be time consuming.

So Isabel told her dad to “check the app store” to see if a fraction calculator already existed.

“There was nothing there that looked reasonable,” Hughes said. “So we said, ‘well, maybe we should make one.’”

Fraction Calculator was born out of that sixth-grade homework assignment.

The application’s simple interface reduces the time and strokes it takes to calculate fractions. It became the platform for Hughes and a business partner, Micah Koffron, to create Calculator Plus.

Both Fraction Calculator and Calculator Plus have enjoyed success. The latter now has had more than 1 million Kindle Fire downloads.  It is the No. 1 calculator app for the Kindle Fire tablet in the U.S. and UK, and second-highest rated app in all categories out of a total of 66,280 apps on Amazon’s Android App store, according to Hughes.

The partnership between Hughes and Koffron started when they worked together at Microsoft. Hughes left his job as a project manager there in 2010 to start his own app company, Digitalchemy. Before leaving, he led the Windows 8 App Store, and his interactions with other app developers at the time contributed to his decision.

“I was going and talking to all these app developers and I thought, ‘Man, they all look like they’re having more fun than me,’” Hughes said. “So I left Microsoft … and basically started this thing out of my house.”

Koffron is still a full-time Microsoft employee, and came along as a part-time addition to Digitalchemy. He often works nights and weekends, and helped Hughes lay the foundation for the two calculator apps.

Before Calculator Plus gained popularity, the two remember looking at a spreadsheet every day with all of the Kindle Fire calculator applications on it. Others were ahead of Calculator Plus, so they decided to offer a free version.

Shortly after that happened, USA Today included Calculator Plus in its “25 Essential Apps,” leading to a large increase in installs. This meant less money per install but, as Koffron pointed out, it hasn’t been a bad thing for Digitalchemy.

“It turns out that free isn’t really free,” he said. “We still get ad revenue (and) we have massively more installs. You don’t get as much revenue per person because the ads don’t pay that much, but it does kind of build up over time.”

Hughes and Koffron are responsible for the programming and development of the two apps, but Isabel gets credit for the initial idea and hours of testing.

“I tested out the apps even past my bedtime,” she said.

Isabel pointed out that many of her friends at school have the app. Neither she, her dad nor Koffron anticipated its success.

“I was really surprised, because we’re just a family working in a basement,” she said.

This wasn’t Isabel’s first technology-related idea, either. When she was 9, and her father was still working at Microsoft, she suggested a new layout for Bing’s image-search engine.

Hughes took her design to the head of Bing’s marketing team, and they implemented it.

“If you go to Bing image search today, it works the way she thought of,” Hughes said.

Calculator Plus and Fraction Calculator are the culmination of Hughes and Koffron’s learning experiences. Digitalchemy has released other apps to the marketplace, but none has come close to the success of these two.

“We learned a lot about what doesn’t work,” Hughes noted.

And perhaps most importantly, they learned that simplicity is what leads to installs for these particular apps.

Hughes and Koffron have received feedback from users, who asked for Calculator Plus to add scientific functions. That’s not what earned it its No. 3 ranking, though.

“We really learned that a successful app is small, it’s simple,” Hughes said. “It focuses on what it’s trying to solve.”

Since Isabel helped her father and Koffron in the development process for Fraction Calculator and Calculator Plus, they want to keep her involved.

And while Isabel doesn’t have plans to become an engineer like dad, she said she wants to keep helping Digitalchemy with ideas and testing — especially when it means staying up past her bedtime.

Pete Treperinas is a student in the University of Washington Department of Communication News Laboratory.