IGCSE Nuclear Physics

I can't seem to figure out the method used to solve this question. Answer is B.
Typically, I can go backwards from the final amount of isotope that remains to the initial amount of isotope at the start of the experiment. But that method doesn't seem to apply here. Please help!

https://imgur.com/8ustNzt

(edited 2 months ago)

Reply 1

Eimmanuel

Study Forum Helper

Original post by S0303

https://imgur.com/8ustNzt

Assume the initial mass of the isotope to be M.

t = 0 ------->------- t = 2.5 ------->------- t = 5.0 ------->------- t = 7.5
M --------->---------- M/2

What is the mass of the isotope remaining at t =7.5? Say the remaining mass is N at t =7.5.

M - N = 70
Note that we can express N in terms of M, so the equation is a single variable equation and can be solved.