It's "commutative property," which says that (for addition/multiplication) order of the operator doesn't matter. For eg, 3 * 5 = 5 * 3.
Associative property (again, of multiplication and addition) means that it doesn't matter how you solve an expression if the same operand is used and some numbers are grouped. For eg. 3 * (5 * 4) = (3 * 5) * 4.
Answer:
Ques 16)
We have to simplify the expression:

Ques 17)

Ques 18)
Let the blank space be denoted by the quantity 'x'.

Ques 19)
Let the missing quantity be denoted by 'x'.

Answer:
(i). Ahsan's Present Age = 16
(ii). Age of Ahsan's Mother when ahsan is 21 years old = 50
Step-by-step explanation:
Let's say, x represents the age of ahsan.
y represents the age of his mother.
The system of equation from the date given is:

Solve them will get us:

Regards: Umer
Statement : Val spent $ 34 for the 2 items
contradiction : Neither of the 2 items that Val bought cost more then $ 15
think about it....if neither of them cost more then 15....lets say they cost 15 each...then there is no way he spent $ 34.....he would spend at most $ 30