Rule: Add the smallest number in the group to the largest number in the group, multiply the result by the amount of numbers in the group, and divide the resulting product by 2.
Suppose we want to find the sum of all numbers from 33 to 41. First, add the smallest number to the largest number.
33 + 41 = 74
Since there are nine numbers from 33 to 41, the next step is
74 x 9 = 666 (see Short Cut 15)
Finally, divide the result by 2.
666 + 2 =- 333 Answer
The sum of all numbers from 33 to 41 is therefore 333.
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