Question

Find the missing number: it is not 10%, or 25%, or 5.5%

Answer 1

let's say the original number is "a".

$\bf \begin{array}{|c|ll} \cline{1-1} \textit{a\% of b}\\ \cline{1-1} \\ \left( \cfrac{a}{100} \right)\cdot b \\\\ \cline{1-1} \end{array}~\hspace{5em}\stackrel{\textit{5\% of "a"}}{\left( \cfrac{5}{100} \right)a}\implies \cfrac{5a}{100} \\\\\\ \stackrel{\textit{"a" increased by 5\%}}{a + \cfrac{5a}{100}\implies \cfrac{105a}{100}}~\hfill \stackrel{\textit{5\% of }\frac{105a}{100}}{\left( \cfrac{5}{100} \right)\cfrac{105a}{100}}\implies \cfrac{525a}{10000}\implies \cfrac{21a}{400}$

$\bf \stackrel{\frac{105a}{100}\textit{ increased by 5\%}}{\cfrac{105a}{100}+\cfrac{21a}{400}}\implies \cfrac{21a}{20}+\cfrac{21a}{400}\implies \cfrac{420a+21a}{400}\implies \cfrac{441a}{400} \\\\\\ 1.1025a\implies 1a+0.1025a\implies \stackrel{\textit{original}}{a} + \stackrel{\textit{10.25\% added}}{\cfrac{10.25}{100}a}$