# 1. "A bacteria population doubles in number every 3 hours. If there are 40 individuals now, how many in 24 hours?" SO I used the geometric series sum formula, with n=8(24/3=8), t1=40, and r=2, but I got 10200 as the answer, instead of what the answer key solves as 10240. What did I do wrong?

2. A runner starts jogging 4 kilometres. He increases his distance by 10% each time he runs. a) the distance he runs on the 8th day b) the total distance he has run after 8 days
I have no idea how to do this.

1. 👍
2. 👎
3. 👁
4. ℹ️
5. 🚩
1. you don't need to sum the series. It doubles 8 times in 24 hours, so the population is 40*2^8.

You'd only need the sum if a new population, double the old one, was added every 3 hours.

The runner problem is just the same. Every day you multiply the distance by 1.1

So, on the nth day he runs 4 * 1.1^(n-1)

The sum is the series where
a = 40
r = 1.1
Sn = 40(1.1^n - 1)/(1.1 - 1)

1. 👍
2. 👎
3. ℹ️
4. 🚩
2. oh I see, thank you Steve! I did everything except I didn't realize the 1.1 part! I did 0.1 instead haha

1. 👍
2. 👎
3. ℹ️
4. 🚩