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Series
First row has 10
Each next row has 4 more than the previous

I think they want you to make a table with all the entries to 20 for the first part

Row ----------- Seats
1 10
2 14
3 18
. .
.
.
20 x

Ok what else

Are you familiar with 'Arithmatic sequence'

a series of numbers with a constant difference between them

Yes but i'm not sure how to apply it in this problem

the general form for this type is:
\[a _{n}=a _{1}+(n-1)*d\]

The chairs in any nth row
\[a _{n}=10+4*(n-1)\]

is the first row number , plus, 4 times the number of rows before n

if you missed something let me know

Is that the whole thing

number of chairs in the nth row

so the 20th row has, n=20
\[a _{20}=10+4*(20-1)\]

Is the answer 86

10 + 19*4