Microsoft billionaire Bill Gates tells teenagers in London he hopes to reach his ultimate goal - making sure all children are protected from life-threatening diseases - within 30 years.

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Bill Gates visited the secondary school as part of the "speakers for schools" initiative designed to inspire young people.

It follows the publication of his annual letter - a plea to the world to help the poorest people.

Tweeting a link to the publication, Mr Gates said: "We cannot tolerate a world in which 1 in 7 people is undernourished, stunted, and in danger of starving to death."

He told the teenagers that his goal was to make sure "every child who grows up avoids life-threatening diseases and have health and nutrition so they can develop to their full potential".

He added: "I think my full goal will take 25 to 30 years... hopefully I have enough time that I'll live to see this ultimate goal."

Read more: Bill Gates - capitalist to philanthropist

Innovation for the future

The key themes of Bill Gates' annual letter are innovation in agriculture, global health, and education.

He talks about the importance of funding research into innovative farming techniques.

"Given the central role that food plays in human welfare and national stability, it is shocking - not to mention short-sighted and potentially dangerous - how little money is spent on agricultural research," he writes

In terms of global health, the achievements of the Global Alliance for Vaccines and Immunisation (Gavi) - which is charged with helping poor countries introduce life-saving vaccines - is top of Bill Gates' agenda.

The Microsoft mogul argues that the money pledged to Gavi will save 4 million lives by 2015, but he warns there are still years of work to be done to introduce diarrhoea and pneumonia vaccines into every country.

The final section of his letter focuses on US education, making sure that all students graduate from high school and that those who want to do a degree, can.

Speaking to the teenagers in east London he described how he had dropped out of school in 1973, hastily adding that he "didn't recommend it".

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Improving lives

The computer pioneer-turned-philanthropist will present his vision for improving lives at the launch of an anti-poverty initiative at the London School of Economics.

He is set to meet individuals who will be trained to engage with a diverse range of communities in an attempt to tackle poverty.

Elisha London, UK director of the Global Poverty Project, said: "We are thrilled that Bill Gates has chosen the launch of the Global Poverty Ambassadors to deliver his annual letter.

"His vision and commitment is an inspiration for these ambassadors who will mobilise their own communities in the fight to end extreme poverty."

Peter Marks, group chief executive of the Co-operative Group, which has sponsored the event, said: "The Co-operative's 6 million customer members can be vital in the fight against global poverty.

"From becoming a Global Poverty Ambassador, to committing to buy Fairtrade products, to resolving to switch to a Co-operative Bank account, there is something we can all do to help."