For a given biased coin, the probability that the outcome of a toss is a head is 0.4. This coin is tossed 1,000 times. Let X denote the random variable whose value is the number of times that head appeared in these 1,000 tosses. The standard deviation of X (rounded to 2 decimal places) is ________.

The number of heads $X$ in $n = 1000$ independent trials follows a binomial distribution $X \sim \text{Binomial}(n, p)$ with parameter $p = 0.4$.

The variance of a binomial distribution is given by:

The standard deviation $\sigma$ is:

Rounding to two decimal places gives 15.49 (accepted range: 15.00 to 16.00).