Weighted Average Mental Math Interview Guide
Weighted average mental math guide for expected value, mixture, portfolio, estimation, and probability-weighted interview calculations.
Candidates who need faster weighted-average calculations.
Weighted averages are not simple averages
A weighted average gives more influence to outcomes with larger probabilities, quantities, or weights. This is the arithmetic behind many expected value answers.
Use complements
With two outcomes, compute one weight and use the complement for the other. This reduces setup errors when probabilities add to one.
Concrete example
If a payoff is 80 with probability 25 percent and 40 with probability 75 percent, the weighted average is 0.25 x 80 + 0.75 x 40 = 50.
Estimate by distance
For two values, the average lies closer to the value with larger weight. This gives a quick sanity check before finishing arithmetic.
Common mistakes
Candidates often average the endpoints and ignore weights. Always write the weight attached to each value before multiplying.
Practice the pattern
Use the LeetQuidity curriculum and calibration to turn this topic into a focused practice plan.