Bottle Argon
A look at what is currently available on eBay
 40 CF WELDING CYLINDER tank bottle FOR ARGON NITROGEN US $120.87 
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 20 CF welding cylinder tank bottle argon helium nitroge US $77.87
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 20 CF WELDING CYLINDER tank bottle nitrogen argon mixes US $76.76
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 The Preservino Professional uses 100% argon technology to preserve open bottles US $42.00
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 Scuba 6 CF bail out pony bottle or argon tank for drysuit with sherwood valve US $95.00
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How old is a bottle of wine if the tritium (H-3) content is 25% that of the new wine?
The half-life of tritium is 12.5 years.
1. 0.25 yr
2. 3.1 yr
3. 25 yr
4. 38 yr
5. 50 yr
Another...
The radioisotope potassium-40 decays to argon-40 by positron emission with a half-life of 1.3 x 10^9 yr. A sample of moon rock was found to contain 78 argon-40 atoms for every 22 potassium-40 atoms. The age of the rock is...
1. 8.1 x 10^-10 yr
2. 2.4 x 10^9 yr
3. 2.8 x 10^9 yr
4. 4.6 x 10^9 yr
5. 6.8 x 10^9 yr
Here's the first one:
"Half-life" means you'll have half the activity after the specified time - in the case of tritium, the half life is 12.5 years.
25% = half of a half
In other words - 2 half lives have gone by to get 0.25
Since tritium's half life is 12.5, the total amount of time would be 12.5 x 2 = 25 (answer c).
The second one is a little trickier because 22/78 is not a whole number of half lives. You need to apply a formula. Go visit the following to see how to start it:
http://www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/Physics/decayrate.htm
