Using information the problem gave us, we can say that the amount of interest earned after one year is:
After 1 year, we will have earned $21. Remember though that we also need to find the amount we will earn after 4 months. Since there are 12 months in a full year, we can say that 4 mohts represents a third of a year. Thus, let's multiply our interest by one-third.
After 4 months, we will have generated $7 and after one year we will have generated $21.