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What is the sum of all multiples of 3 or 5 below 2,200?

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4 votes

Answer:

Explanation:

Three

Sum of all numbers beginning at 0 up to 2200 that are divisible by 3

a = 3

d = 3

L = 2199

Now we need to find out how many there are.

L = a1 + (n - 1)*d

2199 = 3 + (n-1)*3 Subtract 3

2199 - 3 = (n - 1)*3 Divide by 3

2196/3 = n - 1

732 = n - 1 Add 1 to both sides

733 = n

Sum = (a + L)*n /2 Substitute the values

Sum = (3 + 2199)*733/2 Combine and Divide by 2

sum = (1101)*733 Multiply

Sum = 807033

Five

Go through exactly the same steps.

a = 5

L = 2195 ( below 2200. You can't include 2200)

d = 5

L = a + (n-1)*d

2195 = 5 + (n-1)*5

2190 = (n - 1)*5

438 = n - 1

439 = n

Sum = (a + L ) * n

Sum = (5 + 2195)*439/2

Sum = 1100 * 439

Sum = 482900

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The problem is ambiguous, so I'm going to leave the two answers that are there.

Here's the problem.

What do you do with the numbers that are divisible by both? I counted them in both sums, but that may not be correct.

As it stands your answer should 965800 + 482900

Nor does it seem right to count them only in 1 sum. So the answer is given by adding the two numbers above.

User Fitzsimmons
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