Attempting to write out the terms for summation notation

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I need to write out all of the terms for the following: $$\sum_{j=0}^{4}\sum_{k=6}^{8} x_{jk} $$

An example of a previous equation that I had to write out the terms for is: $$\sum_{i=1}^{5}z_{i} $$ With the solution being $${z}_1 +{z}_2 + {z}_3 + {z}_4 + {z}_5$$

I have been using MIT's tutorial for Algebraic Formulations as a resource
https://ocw.mit.edu/courses/sloan-school-of-management/15-053-optimization-methods-in-management-science-spring-2013/tutorials/MIT15_053S13_tut02.pdf

But ... none of the examples in the MIT tutorial have formulas where Sigmas are side-by-side.

I really want to understand how to read/interpret/translate these formulas when I come across them, so any recommendations for tutorials, articles, etc. regarding the subject matter are much appreciated.

Thanks in advance!

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You end up with the sum of 15 terms ({0,1,2,3,4}x{6,7,8}=5x3=15) that are:

$$x_{06}+x_{07}+x_{08}+x_{16}+ x_{17}+x_{18}+x_{26}+x_{27}+x_{28}+x_{36}+x_{37}+x_{38}+x_{46}+x_{47}+x_{48}$$.

Of course, since it's a sum, the order of the terms doesn't matter. You can also chose to increase first the index of the first summation, keeping the second index fixed like: $$x_{06}+x_{16}+x_{26}+x_{36}+x_{46}+x_{07}+x_{17}+\ldots$$

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$x_{06} + x_{07} + x_{08} + x_{16} + x_{17} + x_{18} + x_{26} + x_{27} + x_{28} + x_{36} + x_{37} + x_{38} + x_{46} + x_{47} + x_{48}$