There is often confusion what divisor to use when computing a percentage to express a comparison. For instance, when comparing the exchange rate this year and last year, which do you use as divisor, this year or last year? The answer: always use the basis of comparison as the divisor. If you say, “last year’s rate was x% higher than this year’s,” you are using this year as your basis for comparison, so use this year’s exchange rate as the divisor. If you say, “this year’s rate was y% lower than last year’s,” you are using last year as your basis for comparison, so use last year’s exchange rate as the divisor.

Repeat: use the basis of comparison as your divisor.

Another example: “Mike is x% heavier than Jim.” Since Jim is the basis of comparison, use Jim’s weight as your divisor. Jim is y% lighter than Mike.” Since Mike is the basis of comparison, use Mike’s weight as your divisor.