I have come across this equation in a puzzle and I don't even know what it is, never mind the answer. Can any one help me, please?

Here it is: -

sum = 0

Do n = 123 To 321 Step 3

sum = sum + n

Loop n

Display sum

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- September 21st 2005, 10:58 AMvab123Strange equation
I have come across this equation in a puzzle and I don't even know what it is, never mind the answer. Can any one help me, please?

Here it is: -

sum = 0

Do n = 123 To 321 Step 3

sum = sum + n

Loop n

Display sum - September 21st 2005, 12:17 PMrgep
It's asking for the sum 123 + 126 + 129 + ... + 312.

- September 21st 2005, 12:27 PMvab123Strange equation
Thank you for your help. When I'm faced with equations like that I think my mind just shuts down.

- October 28th 2005, 02:21 PMGamigin
This is fairly simple actually:

There are 67 terms in the sequence: floor((321-123)/3) + 1 = 67

The sequence starts at 123

The sequence ends at 321: 3 * floor((321-123)/3) + 123

The average term is (123 + 321) / 2 = 222

The total is going to be average term * number of terms = 222 * 67 = 14874

I've verified this total with a small piece of Java code that doesn't take the above shortcut:

Code:`long sum = 0;`

for (long test = 123; test <= 321; test += 3) {

sum += test;

}

System.out.println("sum = " + sum);