He didn’t follow PEMDAS which is the an acronym for the order of operations. according to pemdas the first step is parenthesis (which is the p in pemdas) there fore the first step is to add the 1 and the 2 then since there are no exponents you multiply so you multiple 5(3) which is 15 then you can add all the number left to right so 15+2+3= 20 so the answer is 20 hope this helped let me know if you have any questions
