Example 01 | Age-related problem

Let
$A$ = present age of Alfred
$F$ = present age of Franco

$A - 5$ = age of Alfred 5 years ago
$F - 5$ = age of Franco 5 years ago
 

Sum of their ages 5 years ago
$(A - 5) + (F - 5) = A$

$F = 10 \, \text{ yrs old}$           answer
 

Alfred is four times as old as Franco
$A = 4(10)$

$A = 40 \, \text{ yrs old}$           answer