it's 20
the only way you can make it anything other than 20 is inferring some imaginary brackets e.g.
320
How?
4x4(16)+4(20)x4(80)+4(84)-4(80)x4(320)
which is actually (((((4x4)+4)x4)+4)-4)x4
the correct way is to do the multiplication then the addition / subtraction
4x4+4x4+4-4x4=???
+16
+16
+4
-16
the +16 and -16 cancel out so you're left with +16+4 = 20
put it into any decent calculator and you get the same result.