Answer:
There are 15 bees.
Explanation:
Let's call x the total number of bees. There is one fifth of that in one bush, which can be written as:

there is one third on another, which is:

the other one has three times the difference between the previous two:

So, if we add those three quantities plus one single bee that flew away, it all should add up to the total number of bees, which is x. So:

We will solve for x:



We will move the positive x on the right of the equal as a negative one to the left:





We can prove this answer by replacing in the original equation:



