Half life help!

Question

If the half life of Technetium -104 is 18.0 minutes, how much of a 165.0 g sample will be left after 90.0 minutes. Round to the correct number of significant figures and use units (g)

Answer 1

Fraction remaining = 0.5ⁿ where n = # of half lives that have elapsed

n = 90 min x 1 half life/18 min = 5 half lives

F.R. = 0.5⁵ = 0.03125

amount left = 0.03125 x 165.0 g = 5.15625 g = 5.156 g

Answer 2

Given a half life of 18 minutes after 90 minutes this is 5 half lives. So of 165.0 grams we would have 5.16 grams left