What is the smallest sum of money which contains Rs. 1.80, Rs. 9, Rs. 3.6 and Rs. 1.20?

Concept- $(LCM\ of\ \left( {\frac{a}{b},\frac{c}{d},\frac{e}{f}, \ldots } \right) = \frac{{LCM\ of\ \left( {a,c,e, \ldots } \right)}}{{HCF\ of\ \left( {b,d,f, \ldots } \right)}})$

The smallest sum of money will be the LCM of 1.80, 9, 3.6 and 1.2 as LCM would be the number which will be multiple of all given four numbers.

i.e we have to find the LCM of 1.80, 9, 3.6 and 1.2

Or, we have to find the LCM of $(\left( {\frac{9}{5},\frac{9}{1},\frac{{18}}{5}\ and\ \frac{6}{5}} \right))$

$(\begin{array}{l}
\Rightarrow LCM\ of\ \left( {\frac{9}{5},\frac{9}{1},\frac{{18}}{5}and\frac{6}{5}} \right) = \frac{{LCM\ of\ \left( {9,9,18,6} \right)}}{{HCF\ of\ \left( {5,1,5,5} \right)}}\\
\Rightarrow LCM\ of\ \left( {\frac{9}{5},\frac{9}{1},\frac{{18}}{5}and\frac{6}{5}} \right) = 18
\end{array})$

Thus, the answer is 18.