Answer:

Hopefully, this is your desired setup. (I noticed the formula you have to fill in there.)
Have a good day.
Explanation:
Hi.
You could use the percent change formula.
Since we know it is a percent increase then we will do new-old instead of old-new:

x is the new amount of shampoo.
16.5 is the original amount (old) of shampoo.
The percent increase is 30%=0.30 .
So we have the following equation:

We could have found the equation like this:

Subtract 16.5 on both sides:

Divide both sides by 16.5:

By us of symmetric property of equality:
