Simple and Compound Interest for Placement Exams

Simple and Compound Interest for Placement Exams

Simple and compound interest problems are staples of aptitude for placements, especially in banking and finance sector recruitment. Understanding the difference between linear and exponential growth is not only useful for aptitude tests but also for making real-world financial decisions throughout your career.

Simple vs Compound Interest

Simple Interest (SI) is calculated only on the original principal for the entire period — it grows linearly. Compound Interest (CI) is calculated on the principal plus accumulated interest from each previous period — it grows exponentially. For the same principal, rate, and time, CI is always greater than or equal to SI.

Key Formulas

SI equals P times R times T divided by 100. Amount with SI equals P multiplied by 1 plus RT divided by 100. Amount with CI equals P multiplied by 1 plus R divided by 100, raised to the power T. CI equals Amount minus Principal. The difference between CI and SI for exactly 2 years equals P multiplied by R divided by 100, all squared. For half-yearly compounding, substitute R divided by 2 for rate and 2T for time.

Solved Example

Find CI on Rs. 10000 at 10 percent per annum for 2 years. Amount equals 10000 times 1.1 squared, which is 10000 times 1.21, giving Rs. 12100. CI equals 12100 minus 10000, which is Rs. 2100. SI for same values is 10000 times 10 times 2 divided by 100, which is Rs. 2000. Difference equals Rs. 100 — confirmed by the shortcut: 10000 times 0.01 equals 100.

Interview Tips

In placement tests, always read carefully for the phrases compounded annually, half-yearly, or quarterly before applying the CI formula. The 2-year shortcut for the difference between CI and SI saves at least 30 seconds per problem. Using the SI formula when CI is asked is the most common and costly mistake in this topic.