Precalc Exponential growth

Question

The count in a bacteria culture was 300 after 20 minutes and 1400 after 30 minutes. Assuming the count grows exponentially,

What was the initial size of the culture?   

Find the doubling period.   

Find the population after 90 minutes.   

When will the population reach 12000.   

Answer 1

1400 = 300e^10k

4.6667 ≈ e^10k

ln 4.6667 ≈ 10k

1.5404 ≈ 10 k

0.15404 ≈ k

A = a₀e^0.15404t

Can you use this formula to answer the rest?