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. 

31 is prime. and it’s prime factorization is just 31.