Respuesta :

John is 15
Rob is 20
Mark is 30

x = John's age
x + 5 = Rob's age 
2x = Mark's age

x + x + 5 + 2x = 65
4x + 5 = 65
-5           -5
4x = 60
/4     /4
x = 15
15 + 5 = 20
2(15) = 30