Identifying simultaneous equations

If a problem involves two unknowns, one equation is usually not enough to find them.
You need a second equation, and then you solve both at the same time.
These are called simultaneous equations.

To spot such a problem, count the unknowns and the separate facts you are given.
Two unknowns with two facts means two equations.
Each fact you are told becomes one equation.

For example: "Ben is 1 year older than twice Vikas's age, and their ages add to 40."
Let Ben be $B$ and Vikas be $V$.
The two facts give $B = 2V + 1$ and $B + V = 40$, which you solve together.

Once you have both equations, solve them by substitution or elimination.
Substitution puts one equation into the other; elimination adds or subtracts them to cancel a variable.
Either way, you use both equations at the same time.

Two numbers add to $20$ and differ by $4$. How would you set this up?
There are two unknowns, so use two equations: $x + y = 20$ and $x - y = 4$.
Solve them together to find $x$ and $y$.

A shop sells shirts at $20 and pants at $30, and a customer buys 5 items for $120. Why is this a simultaneous-equations problem?
There are two unknowns — the number of shirts $s$ and pants $p$.
Two facts give two equations, $s + p = 5$ and $20s + 30p = 120$, solved together.

A problem has two unknowns but gives only one equation. Can you find both values?
No — with two unknowns you generally need two equations.
One equation alone leaves many possible pairs, so you cannot pin down a single answer.