Solve the pair of linear equations $2x + y = 40$ and $x + y = 30$. If the solution satisfies the equation $y = mx + 6$,find the value of $m$.

$A$ two-digit number is obtained by either multiplying the sum of the digits by $8$ and adding $1$,or by multiplying the difference of the digits by $13$ and adding $2$. Find the number.

$A$ motor boat can travel $30 \, km$ upstream and $28 \, km$ downstream in $7 \, hours$. It can travel $21 \, km$ upstream and return in $5 \, hours$. Find the speed of the boat in still water and the speed of the stream (in $km/h$).

The solution of the pair of equations $x+y-1=0$ and $2x+2y-3=0$ is:

Solve the following pair of linear equations by the method of substitution: $x + 8y = 19$ and $2x + 11y = 28$.

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