A rectangular field has a length of 30 feet and an area of 840 square feet. How much will it cost to purchase a fence to enclose the field if the fence sells for [tex]\$9.99[/tex] per linear foot?

To solve the problem of determining how much it will cost to purchase a fence for the rectangular field, follow these steps:

1. Identify the given information:
- The length of the rectangular field is 30 feet.
- The area of the field is 840 square feet.
- The cost of the fence is [tex]$9.99 per linear foot.

2. Find the width of the field:
- The formula for the area of a rectangle is:
\[
\text{Area} = \text{Length} \times \text{Width}
\]
- We know the area is 840 square feet and the length is 30 feet. Plug these values into the formula to solve for the width:
\[
840 = 30 \times \text{Width}
\]
- Divide both sides by 30 to find the width:
\[
\text{Width} = \frac{840}{30} = 28 \text{ feet}
\]

3. Calculate the perimeter of the rectangle:
- The formula for the perimeter of a rectangle is:
\[
\text{Perimeter} = 2 \times (\text{Length} + \text{Width})
\]
- Substitute the length (30 feet) and width (28 feet) into the formula:
\[
\text{Perimeter} = 2 \times (30 + 28) = 2 \times 58 = 116 \text{ feet}
\]

4. Determine the total cost to purchase the fence:
- Multiply the perimeter of the field by the cost per linear foot of the fence:
\[
\text{Total Cost} = 116 \times 9.99 = 1158.84 \text{ dollars}
\]

Therefore, the total cost to purchase the fence for the rectangular field is $[/tex]1158.84.