To convert the probability $4.22 \times 10^{-7}$ into an ordinary decimal number, we'll follow these steps:

1. $10^{-7}$ means moving the decimal point 7 places to the left.
2. Starting with the number 4.22, move the decimal point 7 places to the left.

Performing the conversion:

$4.22 \times 10^{-7} = 0.000000422$

So, the probability written as a decimal is: $0.000000422$

Would you like further details on this conversion, or do you have any other questions?

Here are five related questions to explore:

1. What is the significance of using scientific notation in probability?
2. How would you express $4.22 \times 10^{-8}$ as a decimal?
3. What does a probability of $0.000000422$ imply in a real-world context?
4. How does this probability compare to the probability of two people sharing a birthday?
5. What are the chances of everyone having a different birthday in a smaller group of 23 people?

Tip: When working with very small or very large numbers, scientific notation helps make calculations and comparisons easier.