Express 1500ml as a percentage of 7.5 litres?

First of all change milliliters to litres.

1500 ml = 1,5 l

Now, 
you know that 7,5 l = 100% (the whole amound)
and you are asking how much is: 1,5 l of the whole, so you got:

7,5 = 100%
1,5 = x%

7,5 x = 150
x = 150 / 7,5
x = 20%

1500 ml is a 20% of the 7,5 litres.