Maths

October 2012 Monthly Maths Challenge

NOTE: The October question is now closed, but you can answer the current question here

Question

Hilary is having a party. The first time that the doorbell rings, 1 guest enters. The second time 3 guests enter. Thereafter on each successive ring, a group that has 2 more persons than the group that had entered on the previous ring. How many guests in all will have arrived after the twentieth ring? 

Ring number

Number who enter

Total number

1

1

1

2

3 4

3

5 9

 

 

20