If the half-life for the radioactive decay of zirconium-84 is 26 minutes and I start with a 175 gram sample, how much will be left over after 104 minutes?

Try this site:

Half-life - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Half-life)

N_t = N_0 \left(\frac {1}{2}\right)^{t/t_{1/2}}

Time after t minutes = Initial quanity times .5 to the time divided by the half life duration

N_t = 175\left(\frac {1}{2}\right)^{\frac{104}{26}}

10.9375g

what you do is divide 104 mintues by 26, to see how many half lifes there are you need to find out. In this case it is 4. so all you do is divide
1st half life: 175/2= 87.5g
2nd half life: 87/2= 43.75g
3rd half life: 43.75/2= 21.875g
finally
4th half life: 21.875/2= 10.9375g

easy peasy!