Percent problems

2010018203

Level: 
B
A protective layer of thickness \(d\) reduces the level of harmful radiation by \(10\%\). Determine what is the percentage decrease in the original level of harmful radiation after passing through the layer of thickness \(3d\). Round the result to the whole percent.
\(73\%\)
\(70\%\)
\(30\%\)
\(27\%\)

2010018204

Level: 
B
The aluminium rod and brass rod have the same length at a given temperature. The material constants of the rods are: \(\alpha_{\mathrm{aluminium}}=24\cdot 10^{-6}\,\mathrm{K}^{-1}\) and \(\alpha_{\mathrm{brass}}=18\cdot 10^{-6}\,\mathrm{K}^{-1}\). Suppose both rods are heated to the same higher temperature. Find the true statement about the extensions of both rods. Round the percentage difference in the rods’ extensions to the whole percent. \[~\] Hint: Solid materials expand upon heating. The rod with the initial length \(l_0\) is upon the temperature increasing by \(\Delta t\) extended by the value \(\Delta l = l_0 \cdot \alpha \cdot \Delta t\), where \(\alpha\) is the material constant (coefficient of linear thermal expansion) that is indicative of the extent to which a material expands upon heating.
The extension of the aluminium rod is by \(33\%\) greater than the extension of the brass rod.
The extension of the aluminium rod is by \(67\%\) greater than the extension of the brass rod.
The extension of the aluminium rod is by \(133\%\) greater than the extension of the brass rod.
The extension of the aluminium rod is by \(33\%\) less than the extension of the brass rod.

9000078905

Level: 
B
A new laptop has been introduced to the marked with the original price \(\$1\: 090\). Soon after the introduction the price has been increased temporarily by \(20\, \%\) and after several months decreased by \(30\, \%\). Find the final price for the laptop. Round your result to the nearest integer.
\(\$916\)
\(\$981\)
\(\$952\)
\(\$930\)