Assume that the original amount in the account is x.
We will use:
+ to indicate addition of amount (increase in the amount of money)
- to indicate subtraction of amount (decrease in the amount of money)
We have two changes:
1- The first week, he earned $80 and deposited them into the account.
This means that he added $80 to the already existing amount.
Therefore:
amount in account = x + $80
The integer representing this increase would be +80
2- The second week, he withdrew $85 from the account.
This means that he removed $85 from the already existing amount
Therefore:
amount in account = (x+$80) - $85
The integer representing the decrease would be -85
3- getting the change that occurred:
change in account = x + 80 - 85 = x - $5
This means that for the two weeks together, the amount of money in the account decreased by $5
Hope this helps :)
