21 billion as how much more than 9.8 billion write it in scientific notation

Bismuth-210 is an isotope that radioactively decays by about 13% each day, meaning 13% of the remaining bismuth-210 transforms into another atom (polonium-210 in this case) each day. if you begin with 233 mg of bismuth-210, how much remains after 8 days?

Angles r and s are complementary. the measure of angle r is 31 degrees. which equation can be used to find angle s?

Varies inversely with x. if y=8.5 when x=-1 find x when y=-1