Question

So I have this problem here:

If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9.
The sum of these multiples is 23.
Find the sum of all the multiples of 3 or 5 below 1000.

And I wrote this here:

def multiples(num, below):
    counter = 1
    z = 0
    while True:
        x = num * counter
        if x < below:
            z += x
        else:
            break
        counter += 1
    return z
below = 1000
print "Multiples of 3: " + str(multiples(3, below))
print "Multiples of 5: " + str(multiples(5, below))
print "Added: " + str(multiples(3, below) + multiples(5, below))

If I set below to 10 I get the right answer, 23

Multiples of 3: 18
Multiples of 5: 5
Added: 23

But when I set it to 1000 I get this:

Multiples of 3: 166833
Multiples of 5: 99500
Added: 266333

And this is supposedly wrong, is there something I’m not getting?

Answer 1

Actually you need to remove multiples of 15 once below 1000, because it will be duplicated in both 3 & 5.. Which does not happens for below 10..

Multiple of 3 & 5  = (multiple of 3 + multiple of 5 - multiple of 15)

So, you can use a Set to store those multiples, to remove the duplicate..