Question 17697
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

<font color=blue>1000 : 15 = 30000 : t</font>
That is the proportion you would require.
To put it more clearly,
15 is to 1000, what 't' is to 30000
{{{ 15/1000 = t/30000 }}}
t=30000*15/1000=30*15=450 defective out of 30,000
<P>
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.<P>
Hope this helps,
Prabhat