Can someone please answer??

Number p (0 <= p <= 1) is the probability that ith person will enter the escalator, hence (1-p) is the probability thag ith person wil not enter the escalator. Because of that, from state dp[i][j], we switch to state dp[i+1][j+1] with probability p, and to state dp[i+1][j] with probability (1-p). That's why we add dp[i][j] multiplied by p or (1-p) to dp[i][j].

Let Z(i, j) denote the probability that there are j people on the elevator after i seconds have passed. From a state (i, j), 2 things can happen:

1. A person steps onto the elevator. This happens with probability p, and takes us to state (i + 1, j + 1). Hence, Z(i + 1, j + 1) equals p·Z(i, j), plus any other probability being added to it, hence z[i+1][j+1] += z[i][j]*p
2. A person doesn't step onto the elevator. This happens with probability 1 - p, and takes us to state (i + 1, j). Similar reasoning as above, we have z[i+1][j] += z[i][j]*(1-p)

Obviously there is an edge case — a person can't step onto the elevator if there are already n people on it — so take care of that.