the ratio of seniors to juniors was 5 to 12. if there were 340 in all, how many were seniors?

5 : 12  means  that we have 5 + 12  =  17 equal parts.....

And the seniors were (5/17)  of this......so....

(5/17) (340)  =

(340 / 17 ) (5) =

20 * 5  =

100 seniors