Decimal addition and subtraction
Paul, the vet, has a scale. His scale is set incorrectly, but works fine otherwise. He places Oliver, the cat, on the scale. It read 10 lbs. Paul then removes Oliver and places Luna, also a cat, on the scale. It reads 14 lbs. Then Paul places Oliver and Luna on the scale at the same time. The scale reads 22.5 lbs. Is the scale set too high, or too low, and by how much?
When Luna is placed on the scale with Oliver, the indicator increases by Luna's actual weight, and therefore, Luna weighs 22.5 - 10 = 12.5 lbs. Thus, the scale is set 14 - 12.5 = 1.5 lbs too high.
This puzzle was taken from "The Inquisitive Problem Solver." The original puzzle features humans weighed in kilograms. It's unusual for Americans to express the weight of a person in kilograms, so I adapted the problem to use pounds instead. If you'd rather use this puzzle with whole number arithmetic, instead of decimal arithmetic, you can easily change the numbers and the context of the problem.