Hi

Let x and (1-x) represent the fractional portion of teachers and students respectively 

 35x + 15(1-x) = 20

        20x = 5

          x = 1/4, are teachers and 3/4 are students.

ratio of teachers to students is 1:3



CHECKING our Answer***

 .25*35 + .75*15 = 8.75 + 11.25 = 20 

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