An airplane travelled a distance of $400 \mathrm{km}$ at an average speed of $x \mathrm{km}/\mathrm{hr}.$ On the return journey, the speed was increased by $40 \mathrm{km}/\mathrm{hr}.$ Write down an expression for the time taken for the onward journey.

An aeroplane traveled a distance of $400 \mathrm{km}$ at an average speed of $\mathrm{x} \mathrm{km}/\mathrm{hr}.$ On the return journey, the speed was increased by $40 \mathrm{km}/\mathrm{hr}.$ Write down an expression for the time taken for:

the return journey.
 

The sum of the ages of a father and his son is $45$ years. Five years ago, the product of their ages (in years) was $124.$ Determine their present ages.

In an auditorium, seats were arranged in rows and columns. The number of rows was equal to the number of seats in each row. When the
number of rows was doubled and the number of seats in each row was reduced by $10,$ the total number of seats increased by $300$. Find:

the number of rows in the original arrangement
 

In an auditorium, seats were arranged in rows and columns. The number of rows was equal to the number of seats in each row. When the number of rows was doubled and the number of seats in each row was reduced by $10,$ then total number of seats increased by $300$. Find the number of seat in the auditorium after re-arrangement.