Suppose the first unit of output required 800 labor hours, and the second required only 640. How many hours will be reduced from the second unit to the fourth

320 hours

Explanation:

160 hours (800 - 640 = 160) was reduced from the first unit of output to the second unit of ouput.

Assuming this reduction in time is constant for successive unit output

Number of hours that would be reduced from the second to the fourth = 160 + 160 = 320 hours