The radioactive decay of T1-206 to Pb-206....?

The radioactive decay of T1-206 to Pb-206 has a half-life of 4.20 min. Starting with 5.00 1022 atoms of T1-206, calculate the number of such atoms left after 66.0 min.
Thanks :)

Answer:
Calculate number of half-lives (66/4.2)

(1/2) to the power of the half-lives elapsed gives the decimal amount of material remaining.

(1/2)^1 = 0.5; (1/2)^2 = 0.25; (1/2)^3 = 0.125; etc.

5.00 x 10^22 times the decimal percent tells you how many atoms remain.
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