$x \%$ of $y + y \%$ of $x = ?$

In an examination,$52 \%$ of the candidates failed in English and $42 \%$ failed in Mathematics. If $17 \%$ failed in both the subjects,then the percentage of candidates,who passed in both the subjects,was

$40 \%$ of $20 \% + 30 \%$ of $25 \% + 50 \%$ of $28 \%$ is equivalent to:

The weights of two iron balls are $3.5 \text{ kg}$ and $7.5 \text{ kg}$. What is the percentage weight of the first ball with respect to the second ball?

In an examination,there are three papers and a candidate has to get $35 \%$ of the total to pass. In one paper,he gets $62$ out of $150$ and in the second $35$ out of $150$. How much must he get,out of $180$,in the third paper to just qualify for a pass?

In a class of $80$ students and $5$ teachers,each student received sweets equal to $15 \%$ of the total number of students,and each teacher received sweets equal to $25 \%$ of the total number of students. How many sweets were there in total?