Hi everyone I need help figuring this out I had like a mini breakdown yesterday and figured instead of wallowing I'd ask. I am having a really hard time figuring out how to solve half lifes.

Heres the questions from Princeton Review Gen Chem section:

Ceslsium-137 has a half life of 30 years. How long will it take for only 0.3g to remain from a sample that had an original mass of 2.4g?

The answer is 90 Years and they gave an explanation. (Since 0.3 grams is 1/8 of 2.4 grams, the question is asking how long it will take for the radio-isotope to decrease to 1/8 its original amount. We know that this requires 3 half-lives, since 1/2x1/2x1/2=1/8. So, if each half-life is 30 years, then 3 half lives will be 3(30)=90 years, choice B.

I have no idea how they got that 0.3 g is 1/8 of 2.4 how did they figure this out? Did they just divide it up to figure that out? I am not going to lie I know there is a formula for half lifes the N=N0(1/2)^T/t(1/2)

Any help helps honestly. I figure it might only be 1-3 questions on the whole test but I know if I see it and get stumped I'll regret not taking the time to learn it. Thanks so much in advance to anyone able to help.