The probability of x successes in n independent trials of the experiment is given by the Binomial probability formula, where n is the total number of trials and p is the probability of success in each trial.
Since n = 9 and p = 0.4, we can calculate the probability of x ≤ 3 successes as follows:
P(X ≤ 3) = P(X = 0) + P(X = 1) + P(X = 2) + P(X = 3)
= (9C0)(0.4)^0(0.6)^9 + (9C1)(0.4)^1(0.6)^8 + (9C2)(0.4)^2(0.6)^7 + (9C3)(0.4)^3(0.6)^6
= 0.17496 + 0.41472 + 0.36608 + 0.04320
= 0.99976
Therefore, the probability of x ≤ 3 successes is 0.99976.