Students who are taking examinations, such as the O and A levels, are using social media and mobile marketplace applications to buy personalised revision notes, or "cheat sheets", from their seniors.

While such cheat sheets - which summarise key concepts and formulae - are not allowed into exam halls, students say the notes narrow the focus of their revision to topics likely to be tested and help them ace the exams.

The notes, sold for between $5 and $20 online and on apps such as Carousell, fit chunks of information onto just a few pieces of paper.

One student, who declined to be named, said the notes help save time during revision and reduce anxiety. "It is easier to buy these notes than to make them. It gives us a gauge of what is important," said the 18-year-old, who is taking his A levels this year.

Such resources, put together by students for quick reference, are not new; but more students are making money through them.

Mr Ace Chua, 19, a Catholic Junior College student who is waiting for his A-level results, is selling his cheat sheets on Carousell.

"These notes omit irrelevant information and cover only the important stuff, so it saves time during last-minute cramming," he said, adding that about 10 students were interested in buying his cheat sheets.

Undergraduates, too, are finding a market for their cheat sheets - which are sometimes allowed in university open-book exams - on these platforms.

Business student Jolin Tan, 23, who has 30 students interested in her notes, said: "It is time-consuming to flip through textbooks and locate what we need during such exams. Many students would rather use cheat sheets, which are easier to refer to."

An education firm, Zookal, has also recently bought up "hundreds of cheat sheets" from top secondary and tertiary students, according to its co-founder Jon Tse, 28.

The company offered to buy cheat sheets - from students who scored at least an A - for $10 each via its Facebook page last month.

Mr Tse said the company decided to make it convenient for users of its video platform to access these resources, as many were already sharing them with others. He declined to reveal how much the firm spent.

He said: "Students often encounter similar problems understanding a concept, so when one student can explain it to another in his own words, it is a powerful learning tool.

"The competitive environment and heavy workload have also forced students to become smarter about the way they use their time."

At least 11 tuition centres, along with some private tutors here, are giving out cheat sheets free to their students.

Economics tutor Anthony Fok, 32, provides his students with about 10 pages of A-level economics notes - with key definitions, diagrams and pointers on topics likely to appear in the exams.

He also teaches students to create these materials. "When they come up with their own cheat sheets, they understand the facts better and know where to find the points that they are looking for," he said.

Some educationists agreed that the process of preparing a cheat sheet can improve students' learning outcomes.

Dr Timothy Chan, director of SIM Global Education's academic division, said: "It can be a thought-provoking educational exercise. It can also boost students' confidence and reduce their exam stress because it provides a way for them to organise their thoughts."

But Dr Chan said such benefits may not apply to those who merely pay for another's notes.

"The creator of the cheat sheets may have selectively included only concepts that were hard for, or important to, him."