If the half life of a given substance is 65 days, how long will it take for a 100-gram sample of the substance to decay until there is only 25 grams of the radioactive material remaining? if a sample of radioactive isotopes takes 60 minutes to decay from 200 grams to 50 grams, what is the half life of the isotope?

If a 500.0 g sample of technetium-99 decays to 62.5 g of technetium-99 remaining in 639,000 years, what is the half life of technetium-99?

answer; /// i believe that the correct answer is (c)

multiple

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i just did this and got a 100.

when a device is used in a circuit in which the voltage is 81 v, the current flowing through the device is 3 a. what is the resistance of the device?

a. 27 ω

b. 40.5 a

c. 243 ω

d. 243 a

the correct answer is a. 27