Question 1195973
x=Ed
y=father
z=grandfather
x+y+z=159 (total=n*average)
(1/2)z+(1/3)y+(1/4)x=65, and 6z+4y+3x=780
z-4=4(x-4), or z=4x-12
so
x+y+4x-12=159
5x+y=171
and
24x-72+4y+3x=780
27x+4y=852
from the other two variable equation multiplied by -4
-20x-4y=-684
so 7x=168
x=24 Ed
y=51 father
z=84 grandfather 
that adds to 159
a quarter of Ed's is 6,a third of father's is 17 and half of grandfather's is 42. That sum is 65.