Well, the ratio of yellow beads to blue beads is 4:5. The best way I think is to guess and check multiplying the ratio by integers. You need between 15 and 20.
4 : 5 ( = 9 too little )
8 : 10 ( = 18 just right)
12 : 15 ( = 27 too much)
So per necklace, 8 yellow beads and 10 blue beads would be needed.
I hope this helps :-)
