Assume that Alex's age is x
Since Johnny's age is three times Alex's age, so Johnny's age is 3 x
Since the sum of their ages is 72, add their ages and equate the sum by 72
[tex]x\text{+3x = 72}[/tex][tex]4x=72[/tex]Divide both sides by 4 to find x
[tex]\begin{gathered} \frac{4x}{x}=\frac{72}{4} \\ x=18 \end{gathered}[/tex]So Alex's age is 18 years
