In a colony, 70 families take a morning paper and 60 families take an evening paper, while 85 families take at least one of the two. How many families take exactly one of the two papers?

Adding 70 and 60 gives 130, which counts every family that takes both papers twice, so the number taking both is 130 − 85 = 45. Those 45 must now be removed from the 85 that take at least one, leaving 85 − 45 = 40 families with exactly one paper. The traps: 45 is the count of families taking both, the halfway figure; 25 is 70 − 45, only the morning-only families; and 15 is 60 − 45, only the evening-only families — the answer needs those two groups added, and 25 + 15 = 40.