Probability Calculator
Combine two independent events, and see the odds over repeated tries.
The rules used
For INDEPENDENT events: P(A and B) = P(A)×P(B), and P(A or B) = P(A) + P(B) - P(A and B), subtracting the overlap so it isn't counted twice. 'At least once in n tries' goes through the back door: 1 - (1 - p)ⁿ, the complement of failing every time.
The gacha/loot-box math
A 25% chance tried 10 times gives a 94.4% chance of at least one success, not 250%. And a '1 in 100' drop attempted 100 times succeeds only 63.4% of the time; the naive 'should have it by now' feeling is the source of endless casino and loot-box grief. These formulas assume independence: one try doesn't change the next.