we know he filled 2/3 of the bucket in 15mins, how long for the "whole", namely 3/3 = whole, bearing in mind that 3/3 = 1, just the same as 7/7 = 1 or 1,000/1,000 = 1, so one whole is always 1 fraction wise.
[tex]\begin{array}{ccll} fraction&mins\\ \cline{1-2} \frac{2}{3}&15\\[1em] \underset{whole}{1}&x \end{array}\implies \cfrac{~~ \frac{2}{3}~~}{1}=\cfrac{15}{x}\implies \cfrac{2}{3}x=15\implies \cfrac{2x}{3}=15 \\\\\\ 2x=45\implies x=\cfrac{45}{2}\implies x=22\frac{1}{2}\qquad \textit{22 minutes and 30 seconds}[/tex]
