Final answer:
The formula that represents l in terms of F and w is l = (F - 21) / 5.
Step-by-step explanation:
The formula that represents l in terms of F and w is l = (F - 21) / 5.
According to the question, it takes the apprentice four times as long, so the apprentice would take 4x hours to complete the job alone.
To find the formula, we need to isolate l in the equation F = 5w + 21. We can do this by subtracting 21 from both sides of the equation to get F - 21 = 5w. Then, divide both sides of the equation by 5 to solve for w and we have l = (F - 21) / 5.