The fathers age = F
Roberts age = R
Grandfathers age = G
The fathers age can be represented by:
F = 7 + 2R
The Grandfathers age can be represented by:
G = 2F - 4
Since the grandfathers age is 70, plug that in for G and solve the equation:
70 = 2F - 4 (add 4 to both sides)
74 = 2F (divide both sides by 4)
37 = F
Now that we have the fathers age, substitute it for the above equation
37 = 7 + 2R (subtract both sides by 7)
30 = 2R (divide both sides by 2)
15 = R
So Robert is 15 years old
