Jeff and his family are driving to visit his grandmother. they have driven about 30 miles, which is 2/3 of the way. how much further do they have to go?

Let us assume the total distance Jeff and his family has to go = x
Distance that have already been covered by Jeff and his family = 30 miles
Then
2x/3 = 30
2x = 30 * 3
2x = 90
x = 90/2
  = 45 miles
So
The further distance Jeff and his family has to go = (45 - 30) miles
                                                                               = 15 miles 
So the distance that is still required to be covered by Jeff and his family to visit his grandmother is 15 miles.