Count discrete objects - Math Exercise

Question

Can someone help me with this problem. My best guess is $600$ chairs by drawing out sample data but I can't prove whether my guess I am correct. I wanted to create a solution/ proof but I do not know how.

The same number of seats has been placed in each row at the theater. Mrs. Abrama's chair is in the third row from the front, and the eighteenth row from the back. Mrs Dave's seat has eight chairs to the left of it and eleven chairs to right. How many chairs are there in the theatre?

Answer 1

Mrs. Abrama's chair is in the third row from the front, and the eighteenth row from the back.

From this we can infer that there are 20 rows in total.

Mrs Dave's seat has eight chairs to the left of it and eleven chairs to right.

That is, there are $8+1+11=20$ chairs in each row.

In total: $20\times 20 = 400$ chairs are in the theatre.

EDIT: thanks for Fabio Somenzi for a small correction.