There are \(\displaystyle{10}^{{16}}\) strings of 16 digits between 0 and 9 (for each of the 16 digits we have 10 choices). Also, 100 million = \(\displaystyle{100}\cdot{10}^{{6}}={10}^{{8}}\). So, the probability that the random entered number is valid is

number of valid numbers/all strings of 16 digits=\(\displaystyle\frac{{10}^{{8}}}{{10}^{{16}}}=\frac{{1}}{{10}^{{8}}}\)

which is almost zero.

number of valid numbers/all strings of 16 digits=\(\displaystyle\frac{{10}^{{8}}}{{10}^{{16}}}=\frac{{1}}{{10}^{{8}}}\)

which is almost zero.