What’s the answers to 15 and 16

Aculture started with 3000 bacteria. after 5 hours it grew to 3900 bacteria. predict how many bacteria will be present after 9 hours

Look at this pattern ; 1,4,9, number 10000 belongs in this pattern . what’s the place of this number?

Mr. vector door is buying two new cowboy hats all together the hats cost $75.12 the second heart cost twice as much as the first hat what is the price of the more expensive at round to the nearest hundredth