Number halfway between 100 and 500?

300 Find the difference (500-100=400) then halve the sum(200). Add the halved sum to the lowest number in the equaion(100) or subtract the halved sum(200)from the largest number in the equation(500)

one:500-100=400 two:400/2=200 Three: 200+100=300 or 500-200=300