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

0 votes
85 views

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
Share this puzzle

## 1 Answer

+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.

answer Oct 15, 2015

Similar Puzzles
0 votes

Let us say there are two natural numbers "L" and "R".
We performed eight operations on these two numbers as

Step-1: L = R
Step-2: L x L = R x L
Step-3: L^2 - R^2 = LR - R^2
Step-4: (L + R)( L - R) = R(L - R)
Step-5: L + R = R
Step-6: R + R = R
Step-7: 2R = R
Step- 8: 2 = 1

What is wrong here?

0 votes

Its a 5 digit number where…
1st digit denotes how many zeroes are there in the number
2nd digit denotes how many ones are there in the number
3rd digit denotes how many twos are there in the number
4th digit denotes how many threes are there in the number
5th digit denotes how many fours are there in the number

+1 vote

What is the four-digit number in which the first digit is one third the second, the third is the sum of the first and second, and the last is three times the second?

+1 vote

A cricket has 6 legs, a squirrel has 4 legs and a spider has 8 legs.

In a local zoo, it was found that the total number of legs is 612. Also, there are equal number of each of the above animals/insects.

Can you find out how many animals are there in the zoo?