PORTSMOUTH — Ryan Costa has a fun math fact.

“This is a good one: What’s the sum of one plus two, plus all the other numbers all the way up to 100?” he asked.

Give up?

“It’s 5,050, and you don’t have to do all that addition; there’s actually a way of pairing numbers up and finding how many pairs there will be,” said Costa, 38, who in February took over ownership of Mathnasium of Portsmouth, the math-only learning center on Potomac Road.

The “1+2+3+4+5 … 100” problem is somewhat legendary in math circles thanks to German mathematician Carl Friedrich Gauss (1777-1855). His schoolmaster, the story goes, once assigned it to him as punishment in class. The smart-alecky Gauss astonished his teacher by coming up with the correct answer within seconds.

“His teacher told him, ‘You must have cheated!’” said Costa.

Gauss didn’t cheat, and students aren’t taught to cheat at Mathnasium, either. Rather, the center helps them understand math by teaching it in a way that makes sense to them, Costa said.

The center is open year round, Monday through Thursday and on Saturday, and is for any student (second grade through college), whether they’re struggling with math or simply want to improve their knowledge of the subject.

“The system really works well with almost everybody,” Costa said. “Obviously if a kid is struggling, we want to see him succeed. But if they’re on par for whatever they’re doing, we can get them ahead. If they’re ahead, we can keep them interested with something that maybe they’re not going to see them in school.”

Costa has been tutoring and teaching math for 16 years, including a stint at Brockton High School. “I’ve been doing tutoring independently for a little while, but to make a living doing that you really have to charge a lot of money. So I saw this as a step up — both in terms of expanding my business, but also I like that I can bring high-quality stuff to people who aren’t necessarily the wealthiest of the wealthy,” said Costa, who holds a bachelor’s degree in mathematics from UMass Dartmouth.

Costa said the techniques used at Mathnasium are far different than what’s applied in a traditional school setting. First of all, each student gets plenty of individualized attention.

“The student-to-instructor ratio is three to one, four to one, whereas a typical classroom it’s going to be 25 to one,” he said, noting there are anywhere from six to eight instructors on board at Mathnasium, with two to four of them present for each 90-minute session. The center’s enrollment ranges between about 40 to 70 students, depending on the time of year, Costa said.

Besides the low student/instructor ratio, each student has his or her own individualized curriculum, Costa said. In a school classroom, he said, while students are at different levels and learn in different ways, math is usually taught the same way for everyone.

“I kind of wish the public schools would catch up with that, but then I’d be out of a job,” he said with a chuckle. “Many years of that is how kids learn to hate math, because you always feel overwhelmed, you always feel dumb.”

Math is different than other subjects in that regard, he pointed out.

“My principal when I worked at Brockton High — he was a former history teacher — said, ‘You can learn about World War II without knowing anything about World War I, but you can’t learn Algebra II without Algebra I.' And that’s one of the things that we do well here; the testing gets right down to what is the most basic thing that this kid needs to learn to get on to the next thing.”

At the same time, he said the center tries to make mathematics fun.

“You have to hire the right people, because that makes all the difference,” he said. “We tell our instructors, ‘You want to be you kid’s favorite instructor.’ Beyond that, we have our awards cabinet; everyone earns an award by getting work done. We play some math games in here, but they also get real breaks where they can play Uno or something like that.”

For more information about Mathnasium of Portsmouth, visit www.mathnasium.com/portsmouth.