Kevin is 3 times as old as Daniel. 4 years ago, Kevin was 5 times as old as Daniel. How old is Daniel now?

Kevin's age = 24
Daniel's age = 8

One variable linear equation is an equation that has a variable and the exponent number is one.
Can be stated in the form:

or
ax + b = c, where a, b, and c are constants, x is a variable
Whereas the two-variable linear equation is a linear equation that has 2 variables and the exponent is one
Can be stated in the form:

x, y = variable
In the statement of the task:
Kevin is 3 times as old as Daniel.
Let Kevin = K, Daniel = D
then the age comparison :
K = 3D ⇒ equation 1
4 years ago, Kevin was 5 times as old as Daniel
then the equation becomes
K-4 = 5 (D-4)
K-4 = 5D-20
K = 5D-16 ⇒ equation 2
Both of these equations are equations that have 2 variables, so the solution can be used in 3 ways:
1. graph system
the solution is the intersection point of the two equations / lines.
2. substitution
the solution is we change one variable into another variable form of an equation, then substitutes that variable in another equation
3. elimination
the solution is to eliminate one variable from the equation system.
to determine the variable x we eliminate the variable y first, or vice versa.
We use the second method: substitution