Explanation
Systems of equations
Juan´s age is 4 times the Peter's age, and
eigth years from now the Juan's age will be 36
so, we can solving a system of equations to solve this.
Step 1
let x represents the Juan´s age
let y represents the Peter´s age
so.
Juan´s age is 4 times the Peter's age,so
[tex]x=4y\rightarrow equation(1)[/tex]and
eigth years from now the Juan's age will be 36
[tex]\begin{gathered} add\text{ 8 to both ages, } \\ x+8=36\rightarrow equation(2) \\ \end{gathered}[/tex]Step 2
solve the equations
a)solve equation (2)
[tex]\begin{gathered} x+8=36 \\ subtract\text{ 36 in both sides} \\ x+8-8=36-8 \\ x=28 \end{gathered}[/tex]it means Juan's age is 28
b) replace the x value into equation (1)
[tex]\begin{gathered} x=4y\rightarrow equation(1) \\ 28=4y \\ \text{divide both sides by 4} \\ \frac{28}{4}=\frac{4y}{4} \\ 7=y \end{gathered}[/tex]hence, the Peter's agte is 7
I hope this helps you