Why not? Because every math system you've ever worked with has obeyed these properties! You have never dealt with a system where a×b did not in fact equal b×a, for instance, or where (a×b)×c did not equal a×(b×c). Which is why the properties probably seem somewhat pointless to you. Don't worry about their "relevance" for now; just make sure you can keep the properties straight so you can pass the next test. The lesson below explains how I kept track of the properties.
Answer:

Step-by-step explanation:
we know that
A relationship between two variables, x, and y, represent a proportional variation if it can be expressed in the form
or 
Let
c ----> the total cost of the boat rental
d ---> is the number of days, the family rents the boat
we know that
Kaya's family spends $105 to rent a boat for 7 days
so
For d=7 days, c=$105
Find the value of the constant of proportionality k

substitute the given values

therefore
The linear direct equation is equal to

Each piece of cake costs $.56 ($.55875)
I divided 8.94 by 16 to find how much one piece costs.
Diagonals of the parallelogram are congruent.