Estimate how many different books in the world and give a 95% confidence interval? How to replicate a 10-side dice with a regular dice? How about 100-side? And what is the expected value of tosses you need to generate a useful result? What is the sum of all the digits for integer from 1 to 1,000,000?
Arithmetic, probability etc
100000-111
How many tons does the ocean weigh?
Similar to what others have stated, make sure you know basic probability, combinatorics, and can do these in your head.
Computing an expectation that involved several fractions with denominator 32 in my head
One part of the final round was about forecasting something using data.
When you rolling a coin for five times, what is the probability to get even number of heads.
Probability theory, rolling dices, expected value
You play rocks, paper, scissors with a friend but he can't play rocks. What is the optimal strategy (the winner get 1$ from the loser) and what expected pay-off does it yield?
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