"There's nothing on this calculator that has fractions," she said.
It's slow to convert the fractions into decimals to use the calculator, then convert the result back to a fraction to double check the answer.
"So I asked my dad, 'is there a fraction calculator on the app store?' " Isabel said. "And we looked in the app store and turns out there wasn't, so we decided to make one."
So the two sat down and penciled out what a fraction app would look like. Then her dad went to work creating the app.
It helps if your dad is former Microsoft employee and software developer.
"So we came up with this idea that you can just type it in the way you see a fraction," said Aidan Hughes.
You don't have to go through all the mental gymnastics of converting it to decimals?
"You can just enter it like '3' for the whole number here and '2' for the numerator and the denominator down here," she said.
The final test: that same sheet of fraction problems that took minutes to do, Isabel did it in 50 seconds.
They put it on various app stores, and it was an instant hit with students, teachers, construction workers, and even quilters.
"Who knew? And they love it because they have to figure out 3 1/2 yards of this fabric," Aidan Hughes.
It's now the second highest rated app of all time on the Amazon Kindle app store -- download 1 million times.
"That's crazy," Isabel said. "I would have never expected that."
The app is called "calculator plus". There's a free version with ads and a 99-cent version without ads.
It's available on all the major app stores for all devices.