How long until the radioactive isotope decays to 1.3×10−2 mg ? ?

Method 1:

Let n be the number of half-lives taken.

(1.3 × 10⁻²)/0.050 = (1/2)ⁿ
log(0.013/0.050) = log(1/2)ⁿ
n log(1/2) = log(0.013/0.050)
n = log(0.013/0.050) / log(1/2)

Time taken = (6.0 h) × [log(0.013/0.050) / log(1/2)] = 11.7 h

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Method 2:

λ = ln(2)/(6.0 h) = ln(2)/6.0 h⁻¹

ln(Aₒ/A) = λt
ln[0.05/(1.3 × 10⁻²)] = [ln(2)/6.0 h⁻¹]t
Time taken, t = 6.0 × ln[0.05/0.013] / ln(2) h = 11.7 h

6.0 hrs x log (0.013 mg / 0.050 mg) / log (1/2) = 11.66 hrs = 12 hrs

You could use a half life calculator
https://www.calculator.net/half-life-calculator.html