Use commas given in question : As In many countries the comma (,) is used instead of the full stop (.) to represent a decimal point. For example “3.5” (three and a half) can be written as “3,5”. So the solution is

11,3 + 15,7 + 3 = 30. OR 11.3 + 15.7 + 3 = 30.

Also as per the logic it is no where mentioned in the question that we can’t use decimal numbers.

(15-1)+(13-3)+(11-5)=30