<PRE><B>Question 359507</B>
You have two unknown numbers in this case. &lt;br&gt;

The mean of a set of numbers is simply the sum total of the set values divided by the number of values in the set.&lt;br&gt;

We will let t be the sum total of the values, and n be the number of values in the set.&lt;br&gt;

So, the mean of a set should be {{{t/n=mean}}}&lt;br&gt;

So, in this case, we are given {{{t/n=120}}} and {{{(t+300)/n=135}}}&lt;br&gt;

We just need to solve this system of equations to find t and n, so:&lt;br&gt;

{{{t=120*n}}} solve for t and plug this into the 2nd equation:&lt;br&gt;

{{{(120*n+300)/n=135}}}&lt;br&gt;

{{{120*n+300=135n}}}&lt;br&gt;

{{{300=135n-120n}}}&lt;br&gt;

{{{300=15n}}}&lt;br&gt;

{{{n=20}}}&lt;br&gt;

Thus, there are 20 numbers in the set!&lt;br&gt;

You can then use n=20 to find t, but in this case it is not necessary.&lt;br&gt;

I hope this helps!&lt;br&gt;</PRE>