Jim is 19
let fathers age be x then mothers age = x - 5 and Jim's age is (x - 5)
Total their ages and equate to 100
x + x - 5 + (x - 5 ) = 100
multiply through by 2 to eliminate the fraction
2x + 2x - 10 + x - 5 = 200
5x - 15 = 200 ( add 15 to both sides )
5x = 215 ( divide both sides by 5 )
x = 43
father is 43 years, mother is 38 and Jim is 19