Question

2. What are the last 2 digits of 1 + 2 + 3 + + 2005 2006​

Answer 1

Let S be the sum. So you have

S = 1 + 2 + 3 + … + 2004 + 2005 + 2006

as well as

S = 2006 + 2005 + 2004 + … + 3 + 2 + 1

Then

2S = (1 + 2006) + (2 + 2005) + … + (2005 + 2) + (2006 + 1)

2S = 2007 + 2007 + … + 2007 + 2007

2S = 2006 × 2007

S = (2006 × 2007)/2

S = 2,013,021

which makes the last two digits 21.