write out the beta-decay for flourine-20
how much original radioactive material remains after a substance has
gone through to half lives?

This also seems like a homework problem....

As for the second half of your question lets think about the term
"half-life" which is, as I am sure you should know, the time in which
one half of a sample decays.  So after one half life....one half the
sample would remain.  A second half life, starting with 1/2 the
original sample will decay half of what remains.  Half of one half
is....

These are important problems to know how to do - they help you to
learn to think criticially, don't write them off so dismissively.

element:Flourine
ISOTOPE:18F 	
HALF-LIFE:110 min. 	 
DECAYMODE: + (97%), E.C. (3%) 	
IMAGING ENERGY keV (%Abundance): 511 (194%)

http://www.wramc.amedd.army.mil/departments/nuclear/RadioPharm/radnuclide.htm

Happy hacking
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