C

So far Johnny has received the grades: $5$, $3$, $3$, $3$, $2$, $2$, $1$, $1$, for the math course in this school term. What another grade he needs to get so that the average of grades is lower than $2.5$? (Assumption: All grades are of the same weight in grading scale of $1$, $2$, $3$, $4$, $5$, where $1$ is the best result.)

In a tank, there are $60$ litres of $2\mathrm{\%}$ saline solution. How much water must evaporate to increase salt concentration into $3\mathrm{\%}$?

A sample of $168$ respondents of a sociological research splits into three categories according to nationality: Czech, Slovak and other. There is four times less Slovak respondents than Czech in the sample. The other nationality reported $85\mathrm{\%}$ less respondents than Czech nationality. Choose the equation that represents the algebraic relation between the nationalities of the sample and leads to evaluation of the number of people of the given nationality.

One and a half of a loaf of bread weighs as same as a quarter of a loaf of bread together with a kilo weight. If it is possible, find how much one loaf weighs. (Assume that all the loaves weigh the same.)