Question

Find the sum of all positive integers less than 1000 ending in 3 or 7.

Answer 1

First, you need to list all of the integers that match the condition. Do you know what these integers are?

Answer 2

Positive integers less than 1000 consist of {1 ... 999}.

This may also be written {001 ... 999}.

So you need to count, {003, 007, 013, ... 987, 993, 997}.

Which is the same result as counting {00, 01, ... 98, 99} twice over. (Once for ??3, once for ??7).

Count the number of integers from 0 and 99 inclusive. Double that.

Answer 3

I see it as
(3 + 13 + 23 + + 993) + (7 + 17 + ... + 997)
two arithmetic series,
there are 100 terms in each series with a= 3 and d = 10 and a=7 and d=10
using :
sum= (n/2)(first + last)

sum = (100/2)(3+993) + (100/2)(7+997)
= 100,000

Answer 4

Thanks, guys!

Answer 5

No cheating >:(