### Brain Teasers

# 30 Numbers

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?

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?

### Hint

Try to find the pattern.### Answer

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.

Hide Hint Show Hint Hide Answer Show Answer

## What Next?

**Solve a Similar Brain Teaser...**

Or, get a random brain teaser.

If you become a registered user you can vote on this brain teaser, keep track of

which ones you have seen, and even make your own.

## Follow Braingle!