Edna = 39 years
Ellie = 40 years
Elsa = 41 years
Edna = n
Ellie = n + 1
Elsa = Ellie + 1 = (n + 1) + 1 = n + 2
Edna + Ellie + Elsa = 120
replace
n + (n + 1) + (n + 2) = 120
3n + 3 = 120
3n = 120 - 3
3n = 117
n = 117 : 3
n = 39
Edna = n = 39 years
Ellie = n + 1 = 39 + 1 = 40 years
Elsa = n + 2 = 39 + 2 = 41 years