Put brackets in the calculation below to make it correct.
15 - 4 x 2 + 1 = 3

If you put brackets around (2+1), your method of working is: 
1) 15-4*(2+1)=3 
2) 15-4*3=3
3) 15-12=3
You don't need any more brackets, as the BIDMAS (brackets, Indices, division, multiplication, addition, subtraction) rule does the rest of the job for you. 
The answer is therefore: 15-4*(2+1)=3