Let's represent the total capacity of the reservoir as R.

Since three-fourths of the reservoir is used, the total amount of petrol used to fill the trucks is (3/4) * R.

We are given that five trucks have the same storage capacity, so let's call the capacity of each truck "t".

The amount of petrol used to fill each truck is simply its storage capacity, which is "t".

Therefore, the total amount of petrol used (3/4) * R needs to be equal to the sum of the petrol used by each truck: 5 * t.

We can set up an equation to find the fraction of the reservoir each truck received:

(3/4) * R = 5 * t

To isolate "t" (the capacity of each truck), we can divide both sides of the equation by 5:

t = (3/4) * R / 5

Now, we're interested in the fraction of the reservoir each truck received, which is "t" divided by the total reservoir capacity "R":

t / R = (3/4 * R / 5) / R

Simplifying the equation, we get:

t / R = (3/4) / 5

Therefore, each truck receives 3/20 of the total capacity of the petrol reservoir.