Total money Marcus has = $28
Money spent to buy a notebook = $3.75
Money left now = 
Money needed to be saved = $11.25
So, the amount of money Marcus can spend = 
Cost of a packet of chips = $1.30
The inequality to determine the maximum number of chips he can buy is:
Let the number of chips Marcus can buy = x
As he cannot spend more than $13 to buy chips so equation becomes:

Solving this we get

Hence, Marcus can buy a maximum of 10 packs of chips and save $11.25