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n(ForL) = n(F) + n(L) - n(F&L) ------ a
Given: n(F) = 60, assume total people =100
to find: n(L) =?
1) n(F&L)=0.1*60 = 6
Two unknown in eq a---- (note we still dont know n(ForL)...) Here i assume that there may be people who own neither F nor L.
2) n(L) - n(F&L) = 40
Not suffecient to find n(L).