He has delivered more than 170 million free lessons on maths and science by creating short videos and posting them on the internet for school students and teachers to use.

Time magazine recently listed him as one of the 100 most influential people in the world, in its annual search for people whose work is likely to have an enduring legacy.

Bill Gates, the co-founder of Microsoft and a fellow philanthropist in education, says Mr Khan's decision to produce an online library of more than 3200 lessons will have an enormous global impact.

"The aspiration of khanacademy.org is to give every kid a chance at a free, world-class education," says Mr Gates, writing in Time. "I've used Khan Academy with my kids, and I'm amazed at the breadth of Sal's subject expertise and his ability to make complicated topics understandable."

School systems in Australia and many other nations are struggling to deal with a chronic shortage of qualified maths teachers. The number of students studying advanced maths in years 11 and 12 has been falling alarmingly.

A recent Productivity Commission report on Australia's school workforce revealed two-thirds of schools have problems staffing maths classes and the situation is expected to deteriorate as older, experienced teachers retire.

A lack of qualified science teachers affects about 24 per cent of secondary science students. The persistent shortage of maths and science teachers is forcing many schools to use "out of field" teachers in those subjects.

One of Australia's leading maths specialists, Melbourne University's Hyam Rubinstein, says innovations such as Mr Khan's website academy will help ease some of the difficulties caused by the shortages.

"A website won't solve all of the problems confronting the teaching of mathematics," says Professor Rubinstein, a former chairman of the Australian Academy of Science's national committee for mathematical sciences.

"But this is a wonderful resource to supplement classroom education. A teacher who hasn't had enough training in mathematics might also be able to use it to assist their classroom teaching."

The Khan Academy is a non-profit website, funded by philanthropic grants. A growing number of universities also have websites offering free tutorials to the public.

Professor Rubinstein says the websites are a welcome alternative to the commercial tutoring industry, which has boomed over the past decade to cater for families wanting extra tuition for their children.

"They're a nice alternative to commercial websites, which tend to have a more limited role of preparing students to pass specific exams. There's a higher goal in these websites: a philosophical approach to show students the power of maths and foster a deeper understanding of that power."

The growing popularity of the Khan Academy recently allowed Mr Khan, 35, to move its headquarters out of his home in Silicone Valley, near San Francisco, to a small office.

His global education venture began almost by accident. Mr Khan graduated from the prestigious Massachusetts Institute of Technology (MIT) and Harvard University, with three degrees in mathematics, electrical engineering and computer science, as well as an MBA.

In 2004, while working as a hedge fund analyst in Boston, he started helping Nadia, his teenage cousin in New Orleans, with her maths homework by posting video tutorials on YouTube. Then Nadia's brothers and other relatives asked Mr Khan to make video tutorials on other maths concepts they were struggling with in school.

As he made more videos on an electronic blackboard and posted them on YouTube, more people worldwide started watching them. By 2009, he quit his hedge fund job to work full-time recording videos and developing the Khan Academy website.

Since 2010 grants of more than $3.5 million from Google, the Bill and Melinda Gates Foundation and other philanthropic donors have enabled Mr Khan to hire specialists to expand the website, especially for non-English speakers and broaden the topics taught.

Earlier this year, Mr Khan gave the MIT commencement address to the institution's graduating students. He told them MIT's decision 10 years ago to put almost all of its lecture notes, exams and other course materials on a new website, OpenCourseWare, and make them freely available to anyone, inspired his decision to do something similar for primary and secondary school students.

"That gave me the clarity to understand what Khan Academy could be — an institution that could reach everyone and transcend ideas of profits and businesses," Mr Khan said.