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October 4, 2015
Posted by **Mike** on Monday, April 4, 2011 at 8:11am.

- algebra -
**MathMate**, Monday, April 4, 2011 at 8:35amYou can find the sum of the first 102 counting numbers using Gauss's method, namely:

"the sum of the first n counting numbers is n*(n+1)/2" whether n is odd or even.

For n = 102, the sum is 102*103/2=10506.

So divide 10506 by 5220 to get a quotient of 2, with a remainder of 66.

Mental calculation tip:

To multiply two numbers close to a hundred can be done in the head as follows, no paper, no calculators:

Let the numbers be (100+x) and (100+y), where x and y are small numbers. Take the example of 102 and 103, then x=2, y=3.

Start with the left most digit, which is a 1.

The next two digits are the sum of x and y, that gives 105.

The next two digits are the product of x and y, that gives 10506, et voilą!

Try with 107*109, that should give 11663.

- algebra - oops -
**MathMate**, Monday, April 4, 2011 at 8:39amForgot to divide 10506 by 2 according to Gauss's rule:

10506/2=5253, divided by 5220 gives a quotient of 1 with 33 as a remainder, and divided by 5250 gives a quotient of 1 and 3 as a remainder.

- algebra -
**Anonymous**, Monday, February 11, 2013 at 5:51pmi have no clue i am tring ti find the answer