450
If he used 1/3 of the stock bottle, then you have
150=1/3x
Now, you multiply both sides by 3, which gives you
450=x
So, originally there were 450 bottles in the stock bottle
Yes, for every value of x, there is only one value of y. This means that y is a function of x.
If 
is the amount of strontium-90 present in the area in year 
, and it decays at a rate of 2.5% per year, then
Let 
be the starting amount immediately after the nuclear reactor explodes. Then
or simply
So that after 50 years, the amount of strontium-90 that remains is approximately
or about 28% of the original amount.
We can confirm this another way; recall the exponential decay formula,
where is measured in years. We're told that 2.5% of the starting amount 
decays after 1 year, so that
Then after 50 years, we have
You can factor out a 5n to get
5n(4m-3)
The answer would be y=1/7x-4
