Answer:
28.6 years
Step-by-step explanation:
If the balance doubles then Ana will have a total of 8400 in her account. The interest earned with thus be;
8400-4200 = 4200
Next, we apply the simple interest formula to determine the duration it will take for this level of interest to be realized;
[tex]I=\frac{PRT}{100}\\ 4200=\frac{4200*3.5*T}{100}\\ 100=3.5*T\\T=28.6 years[/tex]