1. B forms a proportion, this is what I did, i simply 10/18 to 5/9 and i try to see if 5/9 is equivalent to 25/45 and it is. 5/9*5/5= 25/45
2. You multiply 27*4=108 then 108/12=9 so a=12
3. you multiply 16*21=336 then 336/12=28 so b=28
Hope this helps!