Tier 4 Visa Interview

I went to submit my application and documents, also sat for the interview. All was going well until I was asked about my father's salary. In the affidavit copy his yearly income was mention in 3 portions- jib salary, land rent, business profit. I just mentioned about the job salary and rent from land, i did mention about the profit from business but I didnt say the amount. So, instead of 31k pound/year, i said 18k/year.
Is there a high chance that this will affect my visa decision in a negative way?
I am really worried about it, the last thing I want is to get refused for a silly mistake I made in the interview.

I am afraid that the answer will be you will have to wait and see. If the ECO doubts the funding available to you then they will reference your omission and sight that as a concern.
On the other side of the equation-/if the official documents supplied to verify his income are conclusive then your omission is less significant.

Not the reassurance you are seeking but it is impossible to read the mind of an ECO...