if Tim has read 10% of the book, that means he still hasn't read 90% of it yet, and we know that that 90% is 45 pages, so say the book has a total amount of pages of "x", which is 100%.
[tex]\begin{array}{ccll} pages&\%\\ \cline{1-2} 45&90\\ x&100 \end{array}\implies \cfrac{45}{x}=\cfrac{90}{100}\implies \cfrac{45}{x}=\cfrac{9}{10} \\\\\\ 450=9x\implies \cfrac{450}{9}=x\implies 50=x[/tex]
