percentage of life julian has lived with grandparents

18 months =  1.5 years......so....

He didn't live with them for  1.5 / 7.5  = 1/5  = 20%  of his life

So......he must have lived with them the other 80%

Gabe is 9 yrs. old, has lived with grandparents since he was 3 yrs old, what is the percentage he lived with grandparents?

3/9=1/3 of his life-Gabe did not live with his grandparents.

1 - 1/3=2/3 of his life lived with his grandparents. Or,

6/9=2/3 of his life lived with his grandparents-which is the same as above.