how do you do 27?????

Hey there!

To start, the equation needed to represent this problem would be an equation:
y= ab^x

a represents the initial amount. In this case, the initial amount would be 4 billion. It would be represented as 4 since each 1= one billion.

b represents the rate. Because the problem discusses a growth, convert the rate, 1.9% to a decimal and add 1 to it (add one if it is a growth, subtract one if it is a decay). b would be 1.019.

x represents the time in which the population grows or t.

When you piece this together, your equation should look like:
y=4(1.019)^t

or

Choice A.

Hope this helps and have a nice day! Feel free to ask any questions about my work. :)