<PRE><B>Question 891549</B>
I get a Venn diagram ( hard to show it ) that gives me
the following:
Let {{{ P }}} is ALL those who take physics
{{{ P - ( 7 + 5 + 8 ) = P -20 }}}  are those who ONLY take physics
{{{ M = 40 }}} = ALL those who take math
{{{ 31 - ( 3 + 5 ) = 23 }}} are those who take biology but not math
---------------------
Now I can say:
{{{ P - 20 + 40 + 23 = 80 }}}
{{{ P + 43 = 80 }}}
{{{ P = 37 }}}
37 students take physics
check:
{{{ 37 - ( 7 + 5 + 8 ) = 17 }}} take ONLY physics
{{{ 40 - ( 3 + 5 + 7 ) = 25 }}} take ONLY math
{{{ 31 - ( 3 + 5 + 8 ) = 15 }}} take ONLY biology
--------------------------
{{{ 17 + 25 + 15 + 3 + 5 + 7 + 8 = 80 }}}
{{{ 42 + 18 + 20 = 80 }}}
{{{ 80 = 80 }}}
OK
Hope you can draw the 3 interlocking rings 
and put the numbers where they belong.
Good luck
 </PRE>