The standard way to abbreviate "in care of" is "c/o."  As in:
        Ms.Hillary Clinton
        c/o William Jefferson Clinton
        1600 Pennsylvania ave.      (etc., etc.)
The symbol for "per" can be, prehaps, most easily remembered by thinking
of it as "of."  Here's what I have in mind:
Numerically, two-thirds is commonly expressed as "2/3."  As in:
Two-thirds of the players were outfielders.  That's saying that two out of 
three were outfielders.  If we're satisfied with close approximations, we 
could also say thirty-three out of one hundred were outfielders.  This 
would be expressed numerically as 33/100.  You'll recognize that this number 
also amounts to 33% ("thirty-three per cent").  
That's reasonable enough in light of "cent" coming from "centum" with is 
the base for the Latin term for 100.  And "33%" amounts to "thirty-three of 
each hundred."
The proportion still holds if we're talking of 1200 in a group. Thirty-three 
of each hundred of the original group would come to a final total of 400.  
At the other extreme, if we imagine an original group of only 3, then 
thirty-three of each hundred would indicate a final total of only 1.
The same "/" symbol is used in such simplistic operations as expressing, 
briefly, the cost per ounce ("cost/oz.").  If a 6-ounce can costs 27 cents,
the brief expression of the cost/oz. would be 4.5c/oz.  (Can't make a proper
"cents sign" in ASCII.)  Or decent farm land might be priced at "$1200/acre."
                                                                             
There can get to be rather far-out uses of this sort of symbology.  Imagine 
some political scientist assessing the popularity of the voting process in 
some community.  He might say, "It's commonly true that the votes cast 
amount to 0.47/registered voter."  You can see that's rather silly and the 
more standard approach would be to say, "...amount to 47/100 registered 
voters."  But the arithmetic's the same in either case.
As I may already have said, the single most accepted term for the "@" is 
"the commercial 'at'."  Linguistically, "at" in this case is a short form 
of the expression "at the rate of."  You will recognize, then, that these 
two symbols come close to trespassing on each other's territory.  Looking 
on the "commercial 'at'," btw, you'll recognize it's simply a lowercase 
letter a, with the tail extended to encircle the letter.  One of the slang 
terms for this symbol is "the snail."  Also from appearance.
It's quite common, in any case, to apply @ to cases in which the cost of 
a single item is being shown, so the buyer (or someone else) can calculate 
for him-/herself the cost of a larger quantity.  And when a seller feels
that only larger quantities are likely to be sold (and he doesn't want to 
offer a single specimen for the same per-unit price he asks for a bigger 
bunch), he's likely to cite the price as, e.g., "$3.50/doz." or maybe 
"$84/100."  Then the customer who wants to buy only three to test the 
product, for example, will have to strike his own deal.
I feel sure this is more than you really wanted to know.  Sorry.  My oops.
                                                             --ed
--- TBBS v2.1/NM
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