# Homework Help: A word problem

1. Dec 8, 2009

### Jimmy84

1. The problem statement, all variables and given/known data
I just translated this word problem:

The zone of a alga increases every 2 month about the factor 4. How many days does the alga needs to increse into the double? How many days ago has the alga been the half?

2. Relevant equations

3. The attempt at a solution

1.) The first question states

The algae increases by a factor of 4 every 60 days. how many "days" does the alga needs to increse into the double?

So I got 4x = 60 multiplyed 2

x = 30 days

2.) I dont understand the second question.

How many days ago has the alga been the half?

I would appreciate some help, thanks.

2. Dec 8, 2009

### singular

The second part wants you to look backwards. The algae is size x at the moment. 30 days from now, you found that the algae would be 2x in size. Using the fact that the algae will increase by a factor of 4 in size over 60 days, how many days ago was the algae 0.5x in size?

3. Dec 8, 2009

### Jimmy84

I tryed 2x = 30 multiplyed by 0.5

= 7.5 days ago is that right?

thanks

4. Dec 8, 2009

### singular

If the algae increases by a factor of 4 every 60 days, then 60 days prior to the algae being size x (at present), the algae will be 0.25x. Starting from 0.25x, how long will it take for the algae to increase to 0.5x?