When a number is multiplied by 3 and 10 is added to it gives 25, then the number is

Let the number be $x$

It multiplied by $3=3x$

$10$ is added to $3x=3x+10$

Result $=25$ then $3x+10=25$

$\text{'}10\text{'}$ transpose to $R.H.S$

$\begin{array}{l}3x=25-10\\ 3x=15\end{array}$

$\text{'}3\text{'}$ transpose to $R.H.S$

$x=\frac{15}{3}=5$

$\therefore$the number is 5