If one number is thrice the other and their sum is $16$, find the numbers If one number is thrice the other and their sum is $16$, find the numbers.
I tried, 
Let the first number be $x$ and the second number be $y$
Acc. to question  
$$
\begin{align}
x&=3y &\iff x-3y=0 &&(1)\\
x&=16-3y&&&(2)
\end{align}
$$

Let the first number be $x$. 
Let the second number be $y$. 
According to question
$$ \tag{1}
x+y=16 
$$
$$
\tag{2}   
x=3y
$$
So, $x-3y=0 \tag{2}$
Multiply equation $(1)$ by $3$. 
Solve both equations:
$$\tag{1} 3x+3y=48$$
$$\tag{2} x-3y=0$$
$$\tag{1) + (2}4x=48$$
$$\tag{3}x=12$$
Putting in equation $(1)$: 
$$\tag{1} x+y=16$$
$$\tag{1),(3} 12+y=16$$
$$\tag{4}y=16-12$$
$$\tag{5}y=4$$
