A radioactive sample contains 1.75 of an isotope with a half-life of 3.9 days.

What mass of the isotope will remain after 5.6 days? (Assume no excretion of the nuclide from the body.)

5.6 / 3.9 = 1.4359 (this is the number of half-lives elapsed)

(1/2)^1.4359 = 0.3696 (this is the decimal amount remaining)

1.75 times 0.3696 = 0.647 (to three sig figs)

I didn't put a mass unit on the 1.75 since it could have been g or mg. So, just a reminder: put the appropriate mass unit on the answer.