Basically there are 3 addition properties and they are associative property, identity property, and commutative property.
Commutative property is just changing the order of the addends. Example: 26+19+34+21= 21+34+19+26
Identity Property is just adding a 0 to the number that doesnt change anything. Example: 26+19+34+21 +0 = 100
Associative property is just changing the grouping of the addends. Example: (26+19) + (34+21)
M in this case stands for money, while d means dollars and x is the cost of the ice creams
m = d - x(2)
let me know if you have any other questions
:)