1. The problem asks to convert the scientific notation $1.61 \times 10^{-7}$ into an ordinary decimal number. 2. Scientific notation $a \times 10^{n}$ means move the decimal point in $a$ by $n$ places. 3. Since the exponent is $-7$, we move the decimal point 7 places to the left. 4. Starting with $1.61$, moving the decimal 7 places left adds 6 zeros before 161: $$1.61 \times 10^{-7} = 0.000000161$$ 5. So, the ordinary number is $0.000000161$. This is a very small probability, as expected.