The answer is 2, 5, 7, 10, and 14.
Let x be the number of rows. There is not room for more than 20 rows:
x ≤ 20
Since all rows must be equal, when we divide 70 by x, we must get a whole number. So, we must find all divisors of 70 smaller or equal to 20 and they are: 2, 5, 7, 10, and 14.
So, the possibilities are:
2 rows with 35 chairs in each row
5 rows with 14 chairs in each row
7 rows with 10 chairs in each row
10 rows with 7 chairs in each row
14 rows with 5 chairs in each row
