The Prime Minister's office later described the error as "one of those character-forming events". A spokesman said: "The Prime Minister and those responsible for government communication applaud anything that gets up in lights the issues we are seeking to promote."
According to Anita Straker, director of the National Numeracy project, Mr Byers's mistake is by no means unusual.
She says, there is some confusion about what mental arithmetic is in the first place. The most popular view is that it is the same as number facts - times tables or simple doubles and halves.
Actual calculations are a different matter, however. More important, she says, is understanding how arithmetic relates to real life.