the time required in years for an amount of Sr 90 whose half life is 28 year to be reduced by 75%

Question

the time required in years for an amount of Sr 90 whose half life is 28 year to be reduced by 75%

Answer 1

There are several formulas that one can use to solve this type of problem, but I find the easiest to

Fraction remaining = 0.5ⁿ where n is the number of half lives that have elapsed.

Fraction remaining = 0.25 (since the amount is reduced by 75% that leaves 25% or 0.25)

Solving for n: 0.25 = 0.5ⁿ

log 0.25 = n log 0.5

-0.602 = -0.301 n

n = 2 half lives have elapsed

2 half lives x 28 yrs/half life = 56 years