Quote by: echeewhHey <apache>

Following is the worked solution:

Answer **(2) - 1000**

Analyse the series of numbers in the pattern.

Every group/set of 2 numbers ( starting from left ) adds up to 1.

2000 - 1999 = 1

1998 - 1997 = 1

...

4 - 3 = 1

2 - 1 = 1

From 1 - 2000 (inclusive) there are 1000 groups/sets of these 2 numbers (adding to 1)

2000 ÷ 2 = 1000

hence, the sum of these 1000 groups/sets (of 1 each) is **1000**

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Trust this helps.

Do let me know again if this is different from your <Answerkey> or if there's further clarification.

Cheers,

Edward