In order to enclose a garden plot of 81 m2, we purchased boards with

In order to enclose a garden plot of 81 m2, we purchased boards with a width of 30 cm. How many boards are required if the plot is 9 m wide?

Knowing the area of the site and its width, you can find the length of the site using the formula:
a = S: b, where S is area, a is length, b is width

81: 9 = 9 (m) section length.

To enclose the entire site, you need to arrange the boards around the perimeter of the site. Let’s find the length of the fence.
P = (a + b) * 2, where P is the perimeter, a is the length, b is the width

(9 + 9) * 2 = 18 * 2 = 36 (m) length of the fence

Because the length of the board is set in centimeters, we will translate the length of the fence into centimeters:
1m = 100cm, so
36m = (36 * 100) cm = 3600cm

The length of the entire fence is 3600cm, and the length of one board is 30cm, we will find the number of boards:
3600: 30 = 120 (pcs.)

Answer. You will need 120 planks.