Help please? (will post question in one second)

Question

anonymous · Answer

sup girl

anonymous · Answer

haii wuddup :D 
and so here is the question After three half-lives, a sample contains 10 grams of daughter isotopes. How many grams of the parent isotope were present to start?
The formulas are in the attachments, but it tells me how to find the ending about which is what i already have

anonymous · Answer

okay half life means half of the sample at a time

anonymous · Answer

from my understanding if you had 10 grams after 3 half lives that wold be 20 40 80 but thats only an assumption give me one second

anonymous · Answer

so in other words to get it i would do 10^3?

anonymous · Answer

no its like 
parent
/2/2/2

anonymous · Answer

and i'd assume backwards that would be 10*2*2*2

anonymous · Answer

ohhhh okay i gotchu, well 80 is one of the choices >_<

anonymous · Answer

I guess they gave you n already, since they said 3 half lives.

It's probably supposed to be
$$10 = \frac{b}{2^3}$$
Where b is the beginning amount.

anonymous · Answer

give me one second.

anonymous · Answer

You already figured it out. :D
$$10 \times 2^3 = 10 \times 8 = 80$$

anonymous · Answer

ooo okay thank you ^o^ I completely spaced that formula 2 was meant for that haha

anonymous · Answer

Fantastic, glad i could help.