C is thrice as good a workman as D and together they can finish a work in 31 days. In how many days will D alone complete the same work?

$C$ is thrice as good a workman as $D$.

$C$ and $D$ together can complete the work in 31 days. 

Formula Used: 

Total work = Efficiency $$\times$$ time

Solution: 

According to the question;

​$$\frac{\text{Efficiency of C}}{\text{Efficiency of D}} = \frac{3}{1}$$ 

Total work = $$31 \times 4 = 124 \,unit$$ 

Time taken by D alone = $$\frac{124}{1} = \bf 124 \, days$$