Vara interview questions

A company working with exciting tech and solving a very important problem. Relatively quick feedback from the hiring team. Took a couple of days to set up the first interview. First interview with current Venture Development Associate to screen candidates. Good conversation and got some good insight into the role. The second interview with the hiring manager was about a week later. It was a good discussion with smart questions.

I did 2 interviews of 1 hour each with both of the managers, plus a 3 round interview with both of them in which I had to prepare a case study and a presentation. They were happy with me and wanted to offer the position. I accepted it and signed a contract of 20 hours per week. After 3 hours of signing it, they tell me that it wasn´t valid anymore cause I was explained that it was only 6h/week. How works only 6hours per week? Where is the joke? Totally not serious enterprise. It is not serious neither fair to make someone 3 round interviews and prepare a case study + present it for a position of 6 hours a week. Ridicoulous