Sum of Random Variables | Explaining Greenbook Probability Questions

I am explaining every Quant Greenbook probability problem so I can practice for interviews. Sum of Random Variables Problem Assume that X₁, X₂, ..., and Xₙ are independent and identically-distributed (IID) random variables with uniform distribution between 0 and 1. What is the probability that Sₙ = X₁ + X₂ + ... + Xₙ ≤ 1?