Each bracelet is $5, this can be represented as 5b. (Where b is number of bracelets)
Each necklace is $8, this can be represented as 8n. (Where n is number of necklaces)
We know that the total money raised needs to be at least $100, but it can also be more than that.
So we know that 8n+5b will be greater than or equal too 100.
8n+5b ≥ 100
The property that allow us to write

is called the associative property. It states exactly that, in any sum with more than two terms, you are free to substitute any pair of sum with their sum.