To find the percentage increase, we need to calculate the difference between the new and old number of seats, divide that difference by the old number of seats, and then multiply by 100 to express the result as a percentage:
Difference = New number of seats - Old number of seats
Difference = 171 - 140
Difference = 31
Percentage increase = (Difference / Old number of seats) x 100
Percentage increase = (31 / 140) x 100
Percentage increase ≈ 22.1%
Therefore, the approximate percentage increase of the number of seats in the theater is 22.1%.