G'morning David, 

 DW> I am convinced that, geometrically, if Circle = 20 and Total = 36,
 DW> Triple can have any value between 4.0 and 7.3, depending on exactly how
 DW> the circles are arranged.

I follow that...

 DW> So if the teacher had, say, just stated Triple to be 6 instead of 3,
 DW> the thing would have been perfectly possible.

Agreed.

The sticking point then would be how that might be achieved and 
checked by a student...geometrically.

I can now also see easier ways of checking the coverage of any 3 
circles in any 3 locations virtually - and I guess then altering 
location to search for an outcome - but requiring effort well 
beyond normal homework.

I look forward to reading what you get from the teacher....

:-)
 
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