Explanation:

Let B can finish the work in x days,
So, B can finish the work in 1 day $$=\dfrac{1}{x}$$days
As per the question,
If they are working together, then they can finish the work in 20 days.
So, working together, they can finish the work in $$=\dfrac{1}{20}$$ days
A is 50% more efficient than B, so A will finish the work in $$=\dfrac{50x}{100}=\dfrac{x}{2}$$ days
A can finish the work in one day $$=\dfrac{2}{x}$$
C is 40% less efficient than B, so C will finish the work in $$=\dfrac{140x}{100}=1.4x$$ days
C can finish the work $$=\dfrac{1}{1.4x}$$
If they are working together, then they can finish the work in one day $$=\dfrac{1}{x}+\dfrac{2}{x}+\dfrac{10}{14x}=\dfrac{1}{20}$$
$$\Rightarrow \dfrac{14+28+10}{14x}=\dfrac{1}{20}$$
$$\Rightarrow \dfrac{52}{14x}=\dfrac{1}{20}$$
$$\Rightarrow x=\dfrac{52\times 20}{14}=\dfrac{520}{7}$$
Hence, C will finish the work in $$1.4\times \dfrac{520}{7}=104$$ days
Hence, C will be able to finish the work in $$=\dfrac{30 \times 104}{100}=31.2 days \cong 31days$$