Hi Nitish,

It is uncertain to say that if you answer 30 questions you have a sure shot chance. However if you answer 40, then you might have the best chance to crack it.

Based on the ranks secured in IISER 2019 merit list by candidates in JEE Advanced 2019 and KVPY result, the institute will allow 50% seats to candidates. The rest 50% of seats will be reserved for candidates applying through IISER entrance examination. The cutoff of IISER 2019 for all SC/ST candidates and Kashmiri Migrants is minimum 55% marks in 10+2 or equivalent for all boards. You  can get complete information regarding IISER merit list and cutoff 2019 from this article.

For more details on cutoffs of individual IISERs and merit list, do follow this link: 

Https://engineering.careers360.com/articles/iiser-merit-list-and-cutoff

Hope this will help. 

All the best!