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**Cheeta921** Task: explain step-by-step how to find the sum of the number 1 to 2000 that are not divisible by 3 or 7. Be sure your plan includes steps to check your work.

Because you would already know how, I’ll give you the quick version of what I did. (not explaining exactly what each value is)

The only formula that I used was Sn= [n(a1+an)]/2

Step one. Find sum of numbers 1 to 2000 using formula. (Equals 2001000)

Step two, find sum of numbers 1-2000 that are divisible by 3 (Equals 666333)

Step three find sum of numbers 1-2000 that are divisible by 7 (Equals 285,285)

Step four Subtract answers 2 and 3 from 1.

Step five, find sum of numbers 1-2000 that are divisible by 21 and add it back so that you aren’t subtracting the same numbers twice.

To check…

This is where I draw a blank. I have No idea. Please help.