use logic and simple math to determine how long it will take for a sample of potassium- 42 to decay from 8mg to 0.25mg if the half-life of this radioactive isotope is 124 hours
8mg to 0.25 mg means how many half lives.
1 half life = 4 mg remain
2 half lives = 2 mg remain
3 half lives = 1 mg remain
4 half lives = 0.5 mg remain
5 half lives = 0.25 mg remain.
It takes 124 hours for a half life; therefore, it must take 124 x 5 for the 5 half lives.