Question 1158659
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I'll do the first problem to get you started.


The population increases at a rate of 1 new person every 15 seconds
We can write the ratio
1 person: 15 seconds


Multiply both sides by 4 so that "15 seconds" turns into "60 seconds"
Then we can replace "60 seconds" with "1 minute"


1 person: 15 seconds
4*1 person: 4*15 seconds
4 people: 60 seconds
4 people: 1 minute


The population is increasing by 4 people per minute. 


Lastly, multiply both sides by 20 so that "1 minute" turns into "20 minutes"


4 people: 1 minute
20*4 people: 20*1 minute
80 people: 20 minutes


After 20 minutes, there are 80 new people. 
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