# How many addition operations are there, and what is the last number on the blackboard?

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Numbers from 1 to 30 are written on a blackboard.

Add the first two numbers, and append it to the end, then erase the first two numbers.

For example, after the first time, the numbers become 3, 4, 5, ..., 29, 30, and 3.

Repeat this process until you get only one number.

How many addition operations are there, and what is the last number on the blackboard?

posted Jul 15, 2015

## 1 Solution

+1 vote

After each operation, the numbers reduce by one, therefore you need 29 times.

Of course, the last number remaining on the blackboard is the sum of all the numbers from 1 to 30, which is (1+30)*30/2=465.

solution Oct 15, 2015

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