correct answer is: c) 25%

Explanation

Let, a worker salary was 100 rupees.
If it was decrease 20\%, then the salary was \{100-\left(100\times\large\frac{20}{100}\right)\} rupees.
\rightarrow\left(100-20\right) rupees.
\rightarrow 80 rupees.
Let, ‘x’ percent should his salary be increased to get back the previous salary.
According to the question,
\therefore\{80+\left(80\times\large\frac{x}{100}\right)\}\normalsize=100
\rightarrow\left(80+\frac{80x}{100}\right)\normalsize=100
\rightarrow\large\left(\frac{8000+80x}{100}\right)\normalsize=100
\rightarrow\left(8000+80x\right)=10000 _{[cross multiply]}
\rightarrow 80x=\left(10000-8000\right) _{[interchange]}
\rightarrow 80x=2000
\rightarrow x=\large\frac{2000}{80}
\rightarrow x=25
\therefore 25 percent should his salary be increased to get back the previous salary.
Ans: His salary should be increased 25\% to get back previous salary.

Other ways to ask this same math question are:

1. If a worker’s salary is reduced by 20%, what percentage increase is needed to restore it to the previous level?
2. Calculate the percentage increase required to compensate for a 20% decrease in a worker’s salary.
3. By what percentage should a worker’s salary be increased to regain the previous amount after a 20% decrease?