$A$ person,rowing at the rate of $5 \, km/h$ in still water,takes thrice as much time in going $40 \, km$ upstream as in going $40 \, km$ downstream. Find the speed of the stream in $km/h$.

Students of a class are made to stand in rows. If $1$ student is extra in a row,there would be $2$ rows less. If $1$ student is less in a row,there would be $3$ rows more. Find the total number of students in the class.

Solve the following pairs of equations:
$4x + \frac{6}{y} = 15$
$6x - \frac{8}{y} = 14, y \neq 0$

If the lines given by $3x + 2ky = 2$ and $2x + 5y + 1 = 0$ are parallel,then the value of $k$ is

If $x+1$ is a factor of $2x^{3}+ax^{2}+2bx+1$,then find the values of $a$ and $b$ given that $2a-3b=4$.

The cost of $4$ pens and $4$ pencil boxes is $Rs.\, 100$. Three times the cost of a pen is $Rs.\, 15$ more than the cost of a pencil box. Form the pair of linear equations for the above situation. Find the cost of a pen and a pencil box (in $Rs.$).