Answer:
15 ml
Step-by-step explanation:
If we mix 1 part concentrate and 5 parts water, we have 6 parts in total.
To create 90ml medicine, <em>how many "6 parts" are there?</em>
There would be 90/6 = 15 total "6 parts".
Each of the "6 parts's" 1 part is concentrate, so concentrate would be 1 * 15 = 15 ml.
<u>In ratio:</u>

The answer to your question is 10 2/5
The simplest interpretation would go a little something like this:
We know that we want the total donation amount to be more than $7,900, so we can set up this inequality to begin with

Where
D is the total donations raised (in dollars). How do we find D? Well, we just add up the total number of table reservations sold and the total number of single tickets sold. If we let
r stand for the number of reservation tickets and
s stand for the number of single tickets, then we have

So, the inequality representing this situation would be

And that would probably be fine for this problem.
<span><em>Footnote:</em>
</span>Of course, if this were a real-life scenario, we'd need to take some additional details into account: How many tables do we have? How many people can be seated at each table?