Question 1059609
<pre><b>
5+10+15+...+430

Factor out 5

5(1+2+3+...+86)

Now we know there are 86 numbers being added up.

Let's find the sum of 1+2+3+...+86
Call it S:

 S =  1+ 2 +3+ 4+...+82+83+84+85+86
 S = 86+85+84+83+...+ 5+ 4+ 3+ 2+ 1  <-- turn them backwards 
-----------------------------------
2S = 87+87+87+87+...+87+87+87+87+87  <-- adding the two equations
 S = 86*87
 S = 7482
 S = 3741

So 5+10+15+...+430 = 5(1+2+3+...+86) = 5(3741) = 18705

Edwin</pre><b>