To find the total weight of all the ants in the world, we need to multiply the average weight of one ant by the total number of ants.
Given:
- Average weight of one ant = \(3 \times 10^{-6}\) kg
- Number of ants = \(1 \times 10^{16}\)
The multiplication in scientific notation follows the formula:
\[ (a \times 10^m) \times (b \times 10^n) = (a \times b) \times 10^{(m+n)} \]
So we can compute the total weight as follows:
\[ (3 \times 10^{-6}) \times (1 \times 10^{16}) = 3 \times 1 \times 10^{-6 + 16} \]
Calculating the values:
\[ 3 \times 1 = 3 \] \[ -6 + 16 = 10 \]
Thus, the total weight of all the ants is:
\[ 3 \times 10^{10} \text{ kg} \]
So the approximate weight in kilograms of all the ants in the world is:
\[ \boxed{3 \times 10^{10}} \text{ kg} \]