Assuming that it's written as 310 - (20/2) x 5 + 13, then this is how you should work it out:
310 - (20/2) x 5 + 13
310 - 5 x 5 + 13
310 - 25 + 13
285 + 13
298
298 should be your final answer if you're using PEMDAS.
Hope this helps!
310-20= 290
2*5+13
2*5=10+13= 23
so it would be 290/23 which is 12.6086956522
But if there are any parenthesis between 20/2 then it would be
310-(20/2)*5+13
20/2=10
10*5=50
50+13=63
310-63=247 (: Hoped this helps (: