If $2: x :: 5: 7$,then $x$ is equal to

In a class there are $z$ students. Out of them $x$ are boys. What part of the class is composed of girls?

$A$ mixture contains alcohol and water in the ratio of $12: 5$. On adding $14 \text{ litres}$ of water,the ratio of alcohol to water becomes $4: 3$. The quantity of alcohol in the mixture is (in $\text{litres}$):

$A$ sum of money is divided among $C, A$ and $B$ in the ratio of $4:5:6$ and another sum of money is divided between $M$ and $N$ equally. If $B$ got $2000$ more than $M$,then how much does $C$ get (in $Rs.$)?

Two horses cost as much as $5$ dogs,$6$ dogs as much as $8$ oxen,$10$ oxen as much as $50$ sheep,$14$ sheep as much as $9$ goats. If the price of one goat is $Rs. 700$,find the cost of one horse (in $Rs.$).

$A$ person distributes his pens among four friends $A, B, C$ and $D$ in the ratio $\frac{1}{3}: \frac{1}{4}: \frac{1}{5}: \frac{1}{6}$. What is the minimum number of pens that the person should have?