GMAT 数学题（3）

lonelyorchid

Another solution
the number of digits in each row equals to   2*(the order number of the row)-1
so the number of digits in first 20 rows is
(1*2-1)+(2*2-1)+......+(20*2-1)
=(1+2+...+20)*2-20
=400
thus, the initial digit in 21st row is 401

sdcar2010

Another solution
the number of digits in each row equals to   2*(the order number of the row)-1
so the number of digits in first 20 rows is
(1*2-1)+(2*2-1)+......+(20*2-1)
=(1+2+...+20)*2-20
=400
thus, the initial digit in 21st row is 401

-- by 会员 lonelyorchid (2011/1/2 14:59:52)

Good.

This is simply another way to express that
the number of number(s) on each row is an odd number:
number(s) in row n = 2*n-1