He has to run 4 miles to finish the marathon
Here, we want to calculate the number of miles left to finish the marathon
To get this, we shall be subtracting the distance he has ran from the total
kindly note that 1/2 is same as 0.5
Hence 7 1/2 miles is same as 7.5 miles
Mathematically, that would be;
16 miles - (7.5 miles + 4.5 miles)
= 16 miles - 12 miles
= 4 miles