The school swim team has 80 swimmers. the ratio of seventh grade swimmers to all swimmers is 5: 16. and i have to write a proportion that gives the number s of seventh grade swimmers. how do i do this?

Given that the total number of swimmers is 80, and the ratio of the seventh grade swimmers to all swimmers is 5:16. You will know that answer for this through cross multiplication. So let's put it this way:
5        ?
--      
16     80
Multiply 80 by 5 and you get 400 then divide this by 16 and you get 25.
Therefore, out of 80 swimmers, there are 25 seventh grade swimmers. Hope this answer helps. 

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