What are the marks at which it was 10 CGPA in first year at IIT Guwahati?

It’s almost impossible to achieve a 10 CGPA after the first year at IIT Guwahati or any other IIT. So I will assume it’s the GP (Grade Point) you refer to. Well, the GP of every course varies throughout the year

• The lab marks are normalized with your lab group. This means they are calculated relatively. For example, students might get 10 GP for getting 85+ marks.
• In lectures, the relative grading is followed. Under this, every student is given a different grade point. Usually, the rank ranges are like this; Rank 1–35 get 10 GP, Rank 36–100 get 9, and so on. In the case of absolute grading, 75+ or 80+ get 10 GP, 65+ or 70+ get 9, and so on.

The grading system at IIT Guwahati can be seen changing every year depending on the student's performance and professors' overview of the batch.