Startup Idea : competition App

Basically its an app idea that lets anyone enter a competition for no more than a pound or dollar, and they have to complete three tasks, these tasks could be anything from tweeting something to going to see a certain movie( this is were some brand deals/advertising could come into the equation).Anyway you win by simply being the first to complete these tasks, also the prize value would get bigger depending on the tasks and how many people join the competition. I would also like to donate half of my profits each month to a charity of the peoples choice, via a vote on social media. I just would love to find out what the public think of these and if you have any advice or questions i would love to answer them. Thank you .