jmc

The number of workers will be inversely proportional to the time needed to complete the job. This means that the product $(\text{number of workers})\times (\text{days to complete job})$ will be a constant. In this case, that constant will be: $$4\times 1.25=5$$ For three workers, the product will remain the same. Let $D$ equal the number of days needed for three workers to complete the job. Then, $$\begin{array}{rl}3\times D& =5\\ \Rightarrow D& =5/3=\boxed{1\frac{2}{3}}\text{work-days}.\end{array}$$