Do I have a chance of making it to IIM Ahmedabad if I scored 81% in 10th and 62 in 12th science?

To be very honest, considering your class 10th and 12th marks, your chances of making it into IIM Ahmedabad are very bleak especially if you are a general category student. 

IIM Ahmedabad has certain criteria pertaining to class 10th and 12th marks. Under this, it’s necessary for the average of 10th and 12th to be greater than 

• 80% in the Science stream
• 77% in the Commerce stream
• 75% in the Arts and Humanities stream

So, with your academic profile, even a 100 percentile in CAT will not fetch you a call from IIM Ahmedabad. However, you can go for top B Schools like FMS Delhi, SP Jain, MDI Gurugram, etc. that give more preference to CAT scores than par academic performance.