Interactive Brokers Software Engineer Interview Experience — A 1-Hour OA With Math Puzzles and a Progressive Tax Bracket Coding Problem

Last week I got a call from an IB recruiter asking if I wanted to try out for an SDE role. After I confirmed, I quickly got an OA scheduled — one hour.

The OA was split into two parts.

Part 1 was middle-school-level math multiple choice.

1. F(x) = 3x - 1, f(f(x)) = 1, what's x? Answer: 5/9
2. I don't remember this one at all — it should have been a pretty simple question.
3. A square is made up of three equal-area rectangles: blue in the top-left, orange in the bottom-left, and green on the right. The blue rectangle has a height of 2. Find the area of the green rectangle. Answer: 16/3
4. There are two concentric circles, sharing the same center. A chord of length 12 lies inside the outer circle and is tangent to the inner circle. Find the area of the ring between the two circles. Answer: 36π
5. A road is made up of infinitely many segments. The first segment is 1000 meters long, and starting from the second segment, each one is half the length of the one before it. Find the sum of all the segment lengths. Answer: 2000

Part 2 was coding. You could use Java, Python, or C++.

Implement a tax calculator for a progressive marginal tax system. Given a sorted list of tax brackets, where each bracket contains a lower income cutoff and a marginal tax rate, calculate the total tax owed for a given income. Each portion of income should be taxed only at the rate corresponding to its bracket. Return the total tax as a double.

Overall I'd say the OA leaned easy. You need to be careful on the math questions, and the coding question is really just checking whether you handle the marginal tax brackets correctly. Good luck to everyone interviewing!