The answer is 6240 + 5t < 8000.
The 6240 represents the combined weight of the horses, regardless of how much they carry, so this will remain constant and without a variable. There are 5 horses, and it unknown how much weight they will carry, so this is represented as 5t. This total must be less that 8000 pounds, as this is the most the trailer can transport.
Hope this works!
