The sum of the first 40 whole numbers is 780.
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40 * * * * * The correct answer is 6400.
The sum of the first 20 even numbers... is 110
2+4+6+8+10........+40=420
You can use the formula for the sum of an arithmetic series to calculate that quickly. (Of course, you can also type the numbers into Excel, and get the sum - but that doesn't scale well - e.g., it won't work well if you are then asked, for instance, to get the sum of the first million even numbers.)