Find the rate at Simple interest, at which a sum becomes four times of itself in $15$ years?

A man can row upstream $10$ kmph and downstream $20$ kmph. Find the man rate in still water and rate of the stream.

A train is $100$ meters long and is running at a speed of $30$ km per hour. Find the time it will take to pass a man standing at a crossing.

A train covers a distance in $50$ minutes if it runs at a speed of $48$ kmph on an average. Find the speed at which the train must run to reduce the time of journey to $40$ minutes.

A cistern can be filled in $9$ hours but due to a leak at its bottom it takes $10$ hours. If the cistern is full, then the time that the leak will take to make it empty will be?

A woman can complete a work in $7$ days and $10$ children take $14$ days to complete the work. Flow many days will $5$ women and $10$ children take to complete the work?

A starts a business with $₹3500$ and after $5$ months, B joins with A as his partner. After a year, the profit is divided into the ratio $2:3$. What is B's contribution in the capital?

In a college, the ratio of the number of boys to girls is $8:5$. If there are $200$ girls, the total number of students in the college is?

In a mixture $60$ litres, the ratio of milk and water $2:1$. If the this ratio is to be $1:2$, then the quantity of water to be further added is?