It'll probably depend on your PPT and/or Windows version.

With 2003 on XP I can click in the text and then do Insert, Symbol, choose
Mathematical Operators in the Subset area and the symbol for "approximately
equal to" is two rows beneath the Euro symbol at character code 2248
(Unicode/hex)

Or put the cursor where you want the symbol, hold down ALT while you type 0187
on the numeric keypad, then select and format the result as Symbol font.

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Steve Rindsberg, PPT MVP
PPT FAQ:  www.pptfaq.com
PPTools:  www.pptools.com
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