Question

Help, please

Answer 1

Half-life is the taken for half of the nuclei of a substance to decay. Mass plays a good deal here, since it is mostly from the nucleus, so we say that the mass also halves after each half-life.
Hence one half-life is 700M/2= 350M, where M is million.
Within this period, the mass also halves, being 100/2= 50grams.

Answer 2

So?

Answer 3

D?

Answer 4

C.