Just like stocks,bonds are sold on the market. They are commonly known as "fixed income" securities.
When corporations or the government are trying to raise money, they sell those at a price lower than the face value and the face value is just the original price.
The price you pay for them is given as a percent of the face value
Any interest you receive is based on the face value. The following explains how we can use skills such as percents to solve such problems.
Find the cost of 4,000 worth of fixed income, which sold for 80 %
The cost is 3,200
How much interest will they pay you if the annual interest is 4 %?