Lets check,
Now let the proportion of defective balls be t
out of every thousand,15 are defective.
Hence out of 30000,t are defective
1000 : 15 = 30000 : t
That is the proportion you would require.
To put it more clearly,
15 is to 1000, what 't' is to 30000
t=30000*15/1000=30*15=450 defective out of 30,000
Logically,if 15 are defective out of 1000 then out of 30,000 there have to be more defective,so the value of t you obtained as 0.5 is incorrect.Try the sum again and keep practising,I'm sure you'll get it.
Hope this helps,
Prabhat