Answer:
<em>You would need 5,555,556 hours to count 1 billion.</em>
Step-by-step explanation:
<u>Proportions</u>
If you count one number every 20 seconds, then you can count 3 numbers in a minute.
Counting 3 numbers in a minute will allow counting 3*60 = 180 numbers in an hour.
At the same counting rate, if you wanted to count 1 billion numbers, we can calculate the hours needed by dividing:
1,000,000,000 / 180 = 5,555,555.55
Rounding to the nearest hour, you would need 5,555,556 hours to count 1 billion.