XP

If something has a 10% success rate and you attempted that thing 10 times, what's the likelihood that you'd succeed? On first glance, some might answer 100% or maybe even very close to 100%, but that's just not how probability works. In this case, it's easier to calculate the probability of failing all 10 attempts. There's a 90% chance of failing one attempt. There's a 81% (0.9 x 0.9) chance of falling two attempts. Continue this and you'll end up with a 34.87% (0.9^10) chance of failing 10 times.

Thus the success rate would be 65.13% (1 - 0.9^10).

Even at 20 attempts, the success probability would still be below 90% (at around 87.84%).