If this ratio is 4:3, then we can write and solve the following equation of ratios:
4 x + 500
---- = -------------
3 x
From this we get 4x = 3x + 1500, or x = 1500.
1500 people own flip phones and (1500+500) own cell phones:
1500 people versus 2000 people.
Note that 2000/1500 forms the proper ratio: 4/3.