1) g = 6j - 6
2) g + j = 78 or j = 78 - g
substitute eq(2) intp eq(1):
g = 6(78 - g) - 6
g = 468 - 6g - 6
g = 462 - 6g
7g = 462
g = 462/7 = 66 years
using eq(2):
j = 78 - 66 = 12
check using eq(1):
66 = 6(12) - 6
66 = 66
The granfather is 66 years old, Junior is 12 years old.