Find the sum of every even positive integer less than 233 not divisible by 10.
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Solution
We find the sum of all positive even integers less than 233 and then subtract all the positive integers less than 233 that are divisible by 10. 2+4+…+232=2(1+2+…+116)=116⋅117=13572. The sum of all positive integers less than 233 that are divisible by 10 is 10+20+…+230=10(1+2+…+23)=2760. Then our answer is 13572−2760=10812.
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Source: Omni-MATH,
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