Element a has a half-life of 10 days. a scientist measures out 200 g of this substance. after 30 days has passed, the scientist reexamines the sample.
how much element a will remain in the sample?
50 g
25 g
12.5 g
100 g

Why does earth rotate? because earth is formed from cold gases collapsing due to gravity because the matter in the nebula that formed earth was spinning because earth forms more than 99% of the mass of the solar system because the hydrogen atoms inside the nebula fused to form helium