The sum of the land percentages is 58.4 (39.3 plus 19.1). To find out how much of this is in the northern hemisphere divide 39.3 by 58.4 to give an answer of 0.673 or 67.3%.
It’s what we called “problems” when I was at primary school!
Bednobs- because the two halves of the world are of equal size we can just take the mean of the two figures. So to find the land % we can just take the average (mean) of 39.3 and 19.1