Rewrite the question in parts. A percentage is something out of \(\displaystyle 100\textup{ parts}\).

\(\displaystyle (\frac{x}{100})(10) = 12\)

Solve for \(\displaystyle x\).

\(\displaystyle (\frac{x}{10})= 12\)

\(\displaystyle x=120\%\)

\(\displaystyle 128\) is \(\displaystyle 40\%\) of a number we will call \(\displaystyle N\); alternatively, \(\displaystyle 0.40\) multiplied by \(\displaystyle N\) is equal to \(\displaystyle 128\). Set up this equation and solve for \(\displaystyle N\):

\(\displaystyle 0.4 N = 128\)

\(\displaystyle 0.4 N \div 0.4 = 128\div 0.4\)

\(\displaystyle N= 320\)

Set up the proportion statement, where \(\displaystyle p\) is our answer, and solve:

\(\displaystyle \frac{p}{100} = \frac{77}{300}\)

\(\displaystyle \frac{p}{100} \cdot 100 = \frac{77}{300} \cdot 100\)

\(\displaystyle p = \frac{77\cdot 100}{300} = \frac{77}{3} = 25 \frac{2}{3} \%\)

One way to figure out what number is a percentage of a larger number is to convert the percent into a decimal and multiply it by the whole number. Since \(\displaystyle 60\%\) is equal to \(\displaystyle 0.60\), multiply this by \(\displaystyle 90\).

\(\displaystyle 90 \times 0.60=54\)

\(\displaystyle 54\) is your answer.

Taking \(\displaystyle 64%\) of a number is the same as multiplying that number by \(\displaystyle 0.64\). We can find our number, therefore, by dividing \(\displaystyle 8,000\) by \(\displaystyle 0.64\):

\(\displaystyle 8,000 \div 0.64 = 12,500\)

Rewrite \(\displaystyle 125\%\) as a decimal by writing \(\displaystyle 125\) with a decimal point, then shifting it two spaces left:

\(\displaystyle 125.0\% = 1.25\)

Multiply this by \(\displaystyle 125\):

\(\displaystyle 125 \times 1.25 = 156.25\)

In order to convert from a percentage to a decimal, simply remove the percentage and move the decimal place back two spaces or divide by 100.

\(\displaystyle 23.2\% = \frac{232}{100}= 0.232\)

The decimal conversion is:  \(\displaystyle 0.232\)

To increase a number by 25% is to take

\(\displaystyle (100 + 25) \% = 125 \%\) of the number, or, equivalently, multiply it by

\(\displaystyle \frac{125}{100} = \frac{125 \div 25 }{100 \div 25 } = \frac{5}{4}\).

Therefore, we divide 7 by \(\displaystyle \frac{5}{4}\):

\(\displaystyle 7 \div \frac{5}{4} = \frac{7 }{1}\div \frac{5}{4} = \frac{7 }{1}\times \frac{4} {5}= \frac{7\times 4 }{1\times 5} = \frac{28}{5}\)

\(\displaystyle 28 \div 5 = 5 \textup{ R }3\), so

\(\displaystyle \frac{28}{5} = 5 \frac{3}{5}\).

To decrease a number by 25% is to take

\(\displaystyle (100 - 25) \% = 75 \%\) of the number, or, equivalently, multiply it by

\(\displaystyle \frac{75}{100} = \frac{75 \div 25 }{100 \div 25 } = \frac{3}{4}\).

Therefore, we divide 7 by \(\displaystyle \frac{3}{4}\):

\(\displaystyle 7 \div \frac{3}{4} = \frac{7 }{1}\div \frac{3}{4} = \frac{7 }{1}\times \frac{4} {3}= \frac{7\times 4 }{1\times 3} = \frac{28}{3}\)

\(\displaystyle 28 \div 3 = 9 \textup{ R }1\), so

\(\displaystyle \frac{28}{3} = 9 \frac{1}{3}\).

Increasing a number by 30% is equivalent to taking 130% of a number, which in turn is equivalent to multiplying it by 

\(\displaystyle \frac{130}{100} = \frac{130 \div 10}{100 \div 10} = \frac{13}{10}\). 

\(\displaystyle 3 \frac{1}{2}\) increased by 30% is the product of \(\displaystyle 3 \frac{1}{2}\) and \(\displaystyle \frac{13}{10}\):

\(\displaystyle 3 \frac{1}{2} \times \frac{13}{10}\)

\(\displaystyle = \frac{3 \times 2 + 1}{2} \times \frac{13}{10}\)

\(\displaystyle = \frac{7}{2} \times \frac{13}{10}\)

\(\displaystyle = \frac{7 \times 13}{2 \times 10}\)

\(\displaystyle = \frac{91}{2 0}\)

\(\displaystyle 91 \div 20 = 4 \textup{ R }11\), so this quantity is equal to \(\displaystyle 4 \frac{11}{2 0}\), the correct answer.