DAY 06

Comment

Look at the way the patterns get bigger.

Answer

231.

Readers' methods

Uzoma Isichei, Greenwich SE10:
It can be seen that the (x+1) th example = x th example + x.
Therefore if 20 th = 210 then
21 st = 210 + 21 = 231.

Senthil Kumar, Hyderabad, India:
We know that the sum of the first "n" natural numbers is n(n+1) / 2.
By applying the same to this problem, we get
number of dots in 21st pattern is 21 x 22 / 2
= 231.

Stephen W, Peacehave BN10:
When it is number one you add 1 to 0, when number 2 you add 2 to the last answer and so on.
So when you get to the 21st you add 21 to the last answer which was 210, and so you get 231.