Advice for MIT PRIMES

Hello, I will be applying to the next cycle of MIT PRIMES USA (first-time applicant). I don’t know what they are expecting in terms of the problem set. I can do a lot of the past problems (with proofs and correct answers) but I’m never sure if it’s good enough. Are they looking for a specific type of rigor in the solutions, or just correct answers and proofs and explanations?