<PRE><B>Question 824865</B>
&lt;pre&gt;
We seek an arithmetic series of 5 terms with common difference d=4
such that S&lt;sub&gt;5&lt;/sub&gt; = 100

S&lt;sub&gt;n&lt;/sub&gt; = {{{n/2}}}[2a&lt;sub&gt;1&lt;/sub&gt; + (n-1)d]

S&lt;sub&gt;5&lt;/sub&gt; = {{{5/2}}}[2a&lt;sub&gt;1&lt;/sub&gt; + (5-1)(4)]

100 = {{{5/2}}}[2a&lt;sub&gt;1&lt;/sub&gt; + (5-1)(4)]

Multiply through by 2

200 = 5[2a&lt;sub&gt;1&lt;/sub&gt; + (4)(4)]

200 = 5[2a&lt;sub&gt;1&lt;/sub&gt; + 16]

200 = 10a&lt;sub&gt;1&lt;/sub&gt; + 80

120 = 10a&lt;sub&gt;1&lt;/sub&gt;

 12 = a&lt;sub&gt;1&lt;/sub&gt;

Answer = 12+16+20+24+28 = 100

Edwin&lt;/pre&gt;</PRE>