$A$ jar contains $10$ red marbles and $30$ green ones. How many red marbles must be added to the jar so that $60 \%$ of the marbles will be red?

$32.05 \% \text{ of } 259.99 = ?$

The population of a variety of tiny bushes in an experimental field increased by $10 \%$ in the first year,increased by $8 \%$ in the second year,but decreased by $10 \%$ in the third year. If the present number of bushes in the experimental field is $26730$,then the number of bushes in the beginning was:

$125 \%$ of $320 + x \%$ of $125 = 440$. Find the value of $x$.

Two jars contain equal quantities of $40 \%$ alcohol. Swati changed the concentration of the first jar to $50 \%$ by adding extra quantity of pure alcohol. Sonali changed the concentration of the second jar to $50 \%$ by replacing a certain quantity of the solution with pure alcohol. By what percentage is the quantity of alcohol added by Swati more than that replaced by Sonali?

$A$ student multiplied a number by $\frac{3}{5}$ instead of $\frac{5}{3}$. What is the percentage error in the calculation?