Lets get started :)
We are told that Mrs. Thymes is buying mulch for her gardens.
Let us check the price for each option:

3 yd³ for $165
Divide 165 by 3, to find the price per cubic yard:

= $55 per cubic yard

4 yd³ for $240
Do the same:

= $60 per cubic yard

6 yd³ for $300

= $50 per cubic yard

8 yd³ for $360

= $45 per cubic yard
Therefore, we can see that 8 yd³ for $360 is the best price
