Two years ago,the sum of the ages of a mother,father,and two daughters was $40$ years. After three years,what will be the sum of their ages?

The sum of the digits of a two-digit number is $9$. The number obtained by interchanging the digits exceeds the original number by $45$. Find the original number.

For what value$(s)$ of $k$ does the pair of equations $kx + 3y = k - 3$ and $12x + ky = k$ have no solution?

Places $A$ and $B$ are $90\, km$ apart from each other on a highway. $A$ car starts from $A$ and another from $B$ at the same time. If they move in the same direction,they meet in $9$ hours and if they move in opposite directions,they meet in $\frac{9}{7}$ hours. Find the speeds of the cars.

Solve the following pair of linear equations:
$152x - 378y = -74$
$-378x + 152y = -604$

If the sum and the difference of the digit at the ten's place and the digit at the unit's place of a two-digit number are the same,then the digit at the unit's place is ...........