A carpenter spend his 40-hour workweek as follows: 1/4 of his ordering materials; 1/2 of his time doing the wooodwork; and 1/8 of his time talking to customers. The rest of his time devoted to cleanup. What is the approximately percentage of time the carpenter spend doing cleanup?

Then, the time that the carpenter spends cleaning up is (what remains from the 40 hours after subtracting the time he spends in the other activities):

[tex]40h-10h-20h-5h=5h\text{.}[/tex]

The question now is, what percentage does 5h represent in 40h? We need to use the percentage formula:

[tex]\begin{gathered} 5=\frac{\%}{100}\cdot40.\leftarrow\begin{cases}\%=\text{ Percentage that 5 represents in 40}\end{cases} \\ \\ \end{gathered}[/tex]

Solving this equation, we get

[tex]\begin{gathered} 5=\frac{\%}{100}\cdot40, \\ \\ 5=\frac{\%\cdot40}{100}, \\ \\ 100\cdot5=\%\cdot40, \\ \\ 500=\%\cdot40, \\ \\ \frac{500}{40}=\%, \\ \\ \%=\frac{500}{40}, \\ \\ \%=12.5. \end{gathered}[/tex]Answer

The percentage of time that the carpenter spends cleaning up is 12.5%.