Half-life

the half life for uranium-235 is 7.0x108
years.

a. how many half-lives did the sample
go through at the end of 2.8x109
years?

b. how much of a 0.74mg sample of
uranium-235 will remain after 2.8x109
years?

(chemistry)

The content of manganese (mn) in steel was determined spectrophotometrically and with the use of the standard addition method. an unknown sample of mn from a digested steel sample gave an absorbance of 0.185 when analyzed spectrophotometrically. when 5.00 ml of solution containing 95.5 ppm mn was added to 50.0 ml of the unknown steel solution (digested sample), the absorbance was 0.248. calculate the concentration, in parts-per-million (ppm), of mn in the digested steel sample solution.