Since we know that [tex]\text{Simple interest}=\frac{PRT}{100}[/tex], where P is principle amount, R is interest rate and T is time.
Now let us substitute our given information in simple interest formula.
[tex]\text{Simple interest}=\frac{220\cdot4\cdot6}{100}[/tex]
[tex]\text{Simple interest}=\frac{5280}{100}[/tex]
[tex]\text{Simple interest}=52.80[/tex]
Therefore, Mary will earn $52.80 after 6 years.
