Ratio proportion: 1.6 : 0.6 :: 0.6 : x

In a proportion $a:b:: b:x$, $x$ is the third proportional given by $x = \frac{b^2}{a}$. Here, $b = 0.6$ and $a = 1.6$, so $x = \frac{0.6 \times 0.6}{1.6} = \frac{0.36}{1.6} = \frac{3.6}{16} = 0.225$.

Confusing the order of terms or misplacing the decimal point when dividing $0.36$ by $1.6$.

This concept of 'third proportional' ($c = \frac{b^2}{a}$) is frequently tested in ratio and proportion problems in RRB and SSC competitive exams.