Gold is $19$ times as heavy as water and copper is $9$ times as heavy as water. In what ratio should these be mixed to get an alloy $15$ times as heavy as water?

If $78$ is divided into three parts which are in the ratio of $1: \frac{1}{3}: \frac{1}{6},$ the middle part is

The ratio of the numbers of boys and girls of a school with $504$ students is $13: 11$. What will be the new ratio if $12$ more girls are admitted?

Find the triplicate ratio of $7:5$.

The sides of a triangle are in the ratio $1/2 : 1/3 : 1/4$ and its perimeter is $156 \text{ cm}$. The length of the largest side is (in $\text{cm}$):

Before a war,the ratio of tanks to planes in an army was $5:3$. During the war,$1000$ tanks were destroyed and $800$ planes were destroyed. The ratio of tanks to planes became $2:1$. What is the number of tanks after the war?