Exponential Decay Question!?

I don't know how to solve this.

Strontium-90 has a half-life of 25 years. How long would it take 4 mg of it to decay to 0.5 mg?

If someone could show me the steps that would be great!

I have this so far:


But I don't know how to solve for t.



Thank you for those answers, but is it possible to solve without the use of logarithms?

2 Answers

  • Anonymous
    1 decade ago
    Favorite Answer


    Recall that:

    y = abⁿ

    This formula is called Exponential Decay formula if 0 < b < 1

    You substitute the variables correctly. Divide both sides by 4 first.

    1/8 = (1/2)^(t/25)

    Rewrite this with logs.

    log(1/8) = tlog(1/2)/25

    Solve for t.

    log(1/8)/[log(1/2)/25] = t

    t = 75

  • 1 decade ago

    first divide both sides by 4 to get rid of that 4 there. So you'll have:


    now, take the log base (1/2) of both sides. from logarithm properties, this will cancel the (1/2) and remove the exponent, leaving the (t/25).

    To do this on a calculator for the left side of the equation, divide the log(.125) by log(1/2). The answer for this is 3. Now we have:



    So your answer is 75 years. Hope I helped!

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