Let 2a=Alex and Alicia’s age and a=Alex or Alicia’s age
And k=Kevin’s age
a+5=k
2a+k=53
And to solve use substitution since we already know what k is
2a+(a+5)=53
2a+a+5=53
3a+5=53
3a=48
a=16
And just in case you wanted to know if needed it
16+5=k
21=k
Alex is 16, Alicia is 16 and Kevin is 21