Have a look at this and description.
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If you are interested in this project, you can do this and we're gonna sign in the contract. The contract basically says that as follows
"You are building a food tracker app with me.
You are going to be doing the coding, and will get 5% of the company in return for that. You will get an MVP out by around the 22nd of August 2019.
If I want to buy your 10%, I can pay you $30 an hour for all the work that you have put into the app. I can start to pay you a regular salary ($10/hour) at any point, and then those hours won’t need to be paid for later on if I want to buy your shares.
I will help out with the marketing and the design along with getting nutritionists onboard the app."
For the gaurantee, you don't need source code right now, but only MVP.