550 ml is the original amount in addition to the 18% that was given free
We can write this as
550 = 100% + 18% = 118%
550 = 118%
The first step to working this out is to work out what 1% of the drink is equivalent to
To get from 118% to 1%, we must divide by 118
We do exactly the same to the drink
550/118 = 4.661ml
We also know that 18% was given free
4.661 x 18 = 83.898ml Free
83.9ml free
