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Sig·Software Engineer·Technical Phone Screen·Junior

Junior
Jun 2026

Summary

SIG quant researcher interview with a probability geometry problem. Pretty classic overlap question but the setup trips you up if you're not careful about how you model continuous uniform distributions.

Questions Asked (1)

Q1

A purple flower starts blooming at a uniformly random time in a 10-day window and stays in bloom for 4 days. Independently, a red flower starts blooming at a uniformly random time in the same 10-day window and stays for 2 days. What is the probability that both flowers are in bloom at the same time?

Algorithms & Data Structures
Author's notes

I set up the sample space as a 10x10 square where each axis is the start time of one flower, then figured out which region of that square leads to overlap.

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AI HintsAI Generated

Suggested Approach

Model the start times as independent uniform random variables on [0,10]. The flowers overlap if the absolute difference between their start times is less than the sum of their bloom durations (4+2=6) and also the earlier flower hasn't finished before the later starts. Compute the area in the 10x10 square where this condition holds, then divide by 100.

Pro tip: Draw a diagram of the sample space and shade the overlap region. This visual approach makes the probability calculation intuitive and helps avoid off-by-one errors in the inequality.

1. Define random variables

Let X be the purple flower's start time and Y be the red flower's start time, both uniformly distributed on [0,10] and independent.

2. Determine overlap condition

The flowers overlap if the intervals [X, X+4] and [Y, Y+2] intersect. This occurs when X ≤ Y+2 and Y ≤ X+4, i.e., |X - Y| ≤ 4? Wait, careful: The condition is X ≤ Y+2 and Y ≤ X+4, which simplifies to -4 ≤ X - Y ≤ 2. So the difference D = X - Y must be between -4 and 2.

3. Compute probability via geometry

In the 10x10 square of possible (X,Y) pairs, the overlap region is the set where -4 ≤ X - Y ≤ 2. Calculate the area of this region by integrating or using geometric shapes, then divide by 100.

4. Calculate area

The area can be found by subtracting the areas of two triangles where the condition fails: one where X - Y > 2 and one where X - Y < -4. Each triangle has legs of length 8 and 6 respectively? Actually, for X - Y > 2, the region is a triangle with vertices (2,0), (10,0), (10,8) – area = 0.5*8*8 = 32. For X - Y < -4, the region is a triangle with vertices (0,4), (0,10), (6,10) – area = 0.5*6*6 = 18. Total non-overlap area = 50, so overlap area = 100 - 50 = 50. Probability = 0.5.

5. Verify and present

Double-check the inequalities and area calculations. Present the final probability as 1/2 or 50%.

Key Points to Mention

  • Independence of the two start times
  • Uniform distribution on a continuous interval
  • Geometric probability: area of favorable region over total area
  • Interval overlap condition: max(start1, start2) < min(end1, end2)
  • Difference of uniform random variables follows a triangular distribution
  • Symmetry can simplify calculations

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.