Grandpa's age is 6 years less than 6 times juniors age. their sum of their age is 78.

Grandpa is 66, Junior is 12.

Grandpa = 6x Juniors age - 6

Grandpa + Junior = 78

78-J = 6J - 6

78 + 6 = 6J + J

84 divided by 7 and 7J divided by 7.

J = 12, so grandpa must be 66.

6n-6 = Grandpa

n = junior

6n-6+n = 78

+6 + 6

7n = 84

/7 / 7

n = 12

6 (12) - 6 = Grandpa

72-6 = 66

Junior is 12 years old, Grandpa's age is 66 years old