Algebraic expression solving Mr. Jain is 4 times as old as his son. After 10 years he will be twice as old as his son. Find Mr. Jain and his son's present ages.
 A: A direct retranscript of the statement is $$j=4s,\\j+10=2(s+10).$$
By substitution,
$$4s+10=2s+20,$$ giving $5$ and $20$ (and later $15$ and $30$).
A: If we set $j$ as the current age of Mr. Jain and $s$ as the current age of his son, we can then write the statements as two linear equations
\begin{align}
j&=4s\\
(j+10)&=2(s+10)
\end{align}
We can then solve this by substituting $j=4s$ into the second equation 
\begin{align}
(j+10) &= 2(s+10) \\
(4s+10)&=2(s+10) \\
4s+10&=2s+20 \\
2s&=10\\
s &= 5
\end{align}
We can then say that \begin{align}j&=4s \\
j &= 4\times 5 \\
j &= 20 \end{align}
A: How to model the statements:

Mr. Jain is 4 times as old as his son.

$$
J = 4 S \quad (1)
$$

After 10 years he will be twice as old as his son.

$$
J + 10 = 2 (S + 10) \quad (2)
$$
These are linear equations. One way to solve this system is inserting equation $(1)$ into equation $(2$):
$$
4S  + 10 = 2 (S + 10) \iff \\
2 S = 10 \iff \\
S = 5
$$
which then is used with equation $(1)$ to give
$$
J = 4\cdot 5 = 20
$$
