They also calculated 30 per cent faster - and the skill gap remained in tests six months later.
Lead researcher Dr Roi Cohen Kadosh, of Oxford University's department of experimental psychology, said he hoped to help people who are bad at maths.
"The study suggests a safe and cheap way we can improve people's maths with limited intervention," he said.
"We are stimulating the part of the brain that deals with maths, because low numeracy is a problem for many people."
The treatment is painless and non-invasive, he added.
An estimated one in five of us struggles with basic arithmetic, while one in 15 suffers from dyscalculia, which can make tasks such as counting change impossible.
It is unclear how the electrical stimulation works, but brain scans suggest it increases the brain's supply of oxygen and nutrients.
The research, published in journal Current Biology, involved five 45 minute maths sessions, with 25 of the participants given brain stimulation for 20 minutes.
- DAILY MAIL