Originally Posted by

**Jameson** It's ok. I'll walk you through it.

You have the right formula. All you need to do is figure out a_1 and r. a_1 is the starting value so this should be easy to figure out since there are very few numbers. Now for the ratio, you know it's decreasing each time by 75%, but written in decimal form the ratio is .75. You want to find for what n you get a_n is 1 or less than one. So let a_n=1, plug in the other info I just wrote about and solve for n.

Does that make more sense?