Answer:
The real GDP per hour worked to increase if there are diminishing returns by less than $500.
Explanation:
Increase in capital per worker from $15000 to $ 20000 increases real GDP per hour worked by $ 500. If there is diminising return to scale then any amount of further increase in capital per worker (say further to 25000 ) will increase GDP less than $ 500. This is because diminising return implies that as we increase our inputs the quantity of our output goes on diminishing. Here the diminishing return has already started ,therefore addtional unit of output will only decrease due to increase in additional unit of input.
Therefore, The real GDP per hour worked to increase if there are diminishing returns by less than $500.