Question

find the difference of the sum of seven numbers and sum of odd numbers between 10 and 20.

Answer 1

The first odd number between 10 and 20 is 11 and the last one is 19. 

The formula for the sum of a series of numbers is: The first number plus the last number quantity times the number of numbers and that product divided by 2. All you need to do now is to figure out how many odd numbers are between 10 and 20 (hint, you can count them on the fingers of one hand), plug in the numbers, and do the arithmetic. 

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