Chisholm Institute of TAFE senior educator Margo Murphy had heard this lie many times before and knew it was an attempt to disguise innumeracy​.

"They've developed survival skills," she explained.

Adults are heading back to the classroom to learn basic maths skills they failed to pick up in school.

Research shows around 20 per cent of Australian students do not reach a basic proficiency in maths that "will enable them to actively participate in the 21st century workforce and contribute as productive citizens."

According to an OECD survey, Australia is ranked 13th out of 23 countries on adult numeracy, but ranked fourth in literacy.

Most of Ms Murphy's students at Chisholm's Foundation College need to upgrade their maths skills so they can function in everyday life, and compete for new jobs.

Josh Cawse, 24, recently started a certificate that is equivalent to year 10 at the TAFE and has been perfecting his decimals, fractions and multiplication skills.

He left school at the start of year nine because "it wasn't for him" and started a career in construction. Now he wants to join the Defence Force.

"I found I struggled with maths in school. I was always trying to keep up with the fastest in the class. Here it is at my own pace. I am motivated and focused now."

Australian Mathematical Sciences Institute schools manager Janine McIntosh said society's poor attitude towards maths was fuelling adult innumeracy.

"It seems to be quite acceptable to say I'm not good at maths. There's a tolerance towards being weaker mathematically. You wouldn't say that about cricket, swimming or English and the ability to read."

Teachers own insecurities about maths were also contributing to the problem, she said.

Ms McIntosh said around 30 per cent of secondary maths teachers were "out of field" – teaching a subject they had no specialised training in.

"The students may have had seven years in primary school and six years in secondary school with some wobbly teachers. If the kids can't see maths as a valuable skill to have then they just won't do it."

Senior Research Fellow at the Australian Council for Educational Research Dave Tout, who has taught numeracy courses at VET providers, said many adult students took up maths courses after struggling with personal finances. Many new parents had trouble with medication doses, cooking and diet, he said.

Adults studying remedial maths programs were often reasonably successful, and could learn year seven to 10 maths within six months, Mr Tout said.

"That's because they are motivated and have a purpose for learning maths. They see the reason for it and can engage with the material. If we could do that more in our secondary schools, we would have more students realising earlier on that maths is useful."