To find the temperature at 4 p.m., we need to determine how many half-hour intervals have passed from noon to 4 p.m. Each half hour, the temperature drops by 3 degrees.
Since there are 12 half-hour intervals from noon to 4 p.m., we multiply the drop in temperature (3 degrees) by the number of intervals (12).
So, the total temperature drop from noon to 4 p.m. is 3 degrees x 12 intervals = 36 degrees.
To find the temperature at 4 p.m., we subtract the total temperature drop from the initial temperature at noon:
75 degrees - 36 degrees = 39 degrees.
Therefore, the temperature at 4 p.m. is 39°F.