Microsoft co-founder Paul Gardner Allen is the wealthiest bachelor and the 5^th richest man in the world, with an estimated net worth of $21 billion.

He is also the second wealthiest college dropout, after Bill Gates, who is the richest man on Earth.

Allen was born in Seattle on January 21, 1953. At Lakeside School, Paul Allen and friend Bill Gates (who is two years younger) became computer enthusiasts. From Lakeside, Allen went to Washington State University but dropped out after two years to give shape to his and Bill Gates' dream of writing software commercially for 'personal computers'.

They founded Microsoft in Albuquerque, New Mexico in 1975. Allen led a deal for Microsoft, winning a contract to supply their operating system -- called QDOS -- to IBM.

Allen was forced to resign from Microsoft in 1983 after being diagnosed with Hodgkin's Disease, a dangerous form of cancer, but was successfully treated by several months of radiation therapy.

• Next: Allen, an investor and a philanthropist

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