Explain how to add decimals to the thousandths?

And explain how you got it so i can understand future problems : 3

Initially 100 milligrams of a radioactive substance was present. after 6 hours the mass had decreased by 4%. if the rate of decay is proportional to the amount of the substance present at time t, find the amount remaining after 24 hours. (round your answer to one decimal place.)

Which of the binomials below is a factor of this trinominal? x^2-13+30