In a market research project, 20\% opted for Nirma detergent whereas 60\% opted for Surf Blue detergent. The remaining individuals were not certain. If the difference between those who opted for Surf Blue and those who were uncertain was 720, how many respondents were covered in the survey?
(a)1440
(b)1800
(c)3600
(d)Data inadequate
Answer
Answer (as printed): B
Explanation
Let the total number of respondents be $x$. Percentage of uncertain individuals $$\begin{aligned} & =[100-(20+60)] \%=20 \% . \\ & \therefore \quad 60 \% \text { of } x-20 \% \text { of } x=720 \Rightarrow 40 \% \text { of } x=720 \\ & \Rightarrow \quad \frac{40}{100} x=720 \Leftrightarrow x=\left(\frac{720 \times 100}{40}\right)=1800 . \end{aligned}$$
Explanation as extracted from the printed page; notation may be imperfect.