Jamieson, Fausset and Brown on Numbers 31:25-47

25-39. Take the sum of the prey that was taken--that is, of the captives and cattle, which, having been first lumped together according to ancient usage (Ex 15:9; Jud 5:30), were divided into two equal parts: the one to the people at large, who had sustained a common injury from the Midianites and who were all liable to serve: and the other portion to the combatants, who, having encountered the labors and perils of war, justly received the largest share. From both parts, however, a certain deduction was taken for the sanctuary, as a thank offering to God for preservation and for victory. The soldiers had greatly the advantage in the distribution; for a five-hundredth part only of their half went to the priest, while a fiftieth part of the congregation's half was given to the Levites.