First, compute how far the family can go in one full tank of gas:
Distance traveled with the first full tank = 28 * 20 = 560 miles
The first stop will be at 560 miles where they will refill the tank.
The second stop will be at 560 + 560 = 1120 miles where they will refill again.
The last stretch will be:
1150 - 1120 = 30 miles from the second stop
They would not need to gas up after the second stop, so there will be 2 stops only.