We follow PE(MD)(AS) rules.  We do just one thing and copy everything
else over.

20-[9-{7+(2*3)}+5]

The innermost parentheses is (2*3)
Replace only the (2*3) with 6 and copy everything else over.

20-[9-{7+6}+5]

Now the innermost parentheses is (7+6)
Replace only the (7+6) with 13 and copy everything else over.

20-[9-13+5]

Now the innermost parentheses is [9-13+5]. But it contains three
numbers to operate with. Do only the leftmost of the addition or
subtraction.  That is, replace only the 9-13 with -4 and copy 
everything else over.

20-[-4+5]

The innermost parentheses is [-4+5]
Replace only the [-4+5) with 1 and copy everything else over.

20-1

19.

Edwin