In a two-digit number, the digit at the unit's place is $x$ and the digit at the ten's place is $2x$. Then, the number is..........

There are some $25$ paise coins and some $50$ paise coins in a bag. The total number of coins is $100$ and the total amount in the bag is Rs. $42.50$. Find the number of coins of each value in the bag.

Devarshi got thrice the marks as obtained by Maharshi in the annual examination of mathematics of standard $10$. The sum of the marks obtained by them was $150$. Represent this situation as a pair of linear equations in two variables.

Two years ago,Salim was thrice as old as his daughter and six years later,he will be four years older than twice her age. How old are they now? (in $year$)

The solution set of a pair of equations $x=6$ and $y=4$ is .........

The pair of equations $2x + 3y = 5$ and $2x + 3y = 8$ is ..........