If a pair of equations $3x + ky - 9 = 0$ and $x + 2y - 3 = 0$ has infinite solutions,then $k = \ldots$

In a two-digit number,the digit at unit's place is $x$ and the digit at ten's place is $y$. Then the number is $\ldots \ldots \ldots \ldots .$

Five years ago,the sum of the ages of five persons was $50$ years. After five years,the sum of the ages of those five persons will be $\ldots \ldots \ldots \ldots$ years.

The present age of Ramesh is $x$ years. Mahesh is $5$ years elder than Ramesh and three years younger than Suresh. The present age of Suresh is $\ldots \ldots \ldots \ldots$

The cost of $2\,kg$ of apples and $1\,kg$ of grapes on a day was found to be Rs. $160$. After a month,the cost of $4\,kg$ of apples and $2\,kg$ of grapes is Rs. $300$. Represent this situation as a pair of linear equations in two variables.

Solve the following pair of linear equations:
$\frac{x}{a} + \frac{y}{b} = a + b$
$\frac{x}{a^{2}} + \frac{y}{b^{2}} = 2, \quad a, b \neq 0$