Let's analyze the situation step-by-step:

Given Information:

• A worker can either cut grass or wash windows in a day.
• It takes x hours to mow a one-acre lawn.
• It takes 2 hours to wash all the windows in one house.
• The worker chooses to wash the windows of 4 houses.

Step 1: Calculate total time spent washing windows

• Time to wash windows in one house = 2 hours
• Time to wash windows in 4 houses = 4 × 2 = 8 hours

Step 2: Calculate how many acres of grass could have been mowed in 8 hours

• Time to mow 1 acre = x hours
• Acres mowed in 8 hours = 8 / x acres

Step 3: Define the opportunity cost

Opportunity cost is the value of the next best alternative foregone. Here, the worker gave up mowing grass to wash windows.

• The opportunity cost of washing windows for 4 houses = the amount of grass that could have been mowed in 8 hours = 8 / x acres.

Final answer:

The opportunity cost of washing the windows of four houses is the amount of grass the worker could have mowed in the same time, which is 8 divided by x acres (8/x acres). If you provide the exact value of x (hours to mow one acre), I can calculate the numerical opportunity cost for you!