How can you avoid spending more than what is in your bank account?
Check your bank statement once a month.
Ask your financial institution to notify you when you
are close to $0 in your account.
Keep your own records to compare with your
financial institution's records.
None of the above

Check your bank statement once a month.

Ask your financial institution to notify you when you  are close to $0 in your account.

Keep your own records to compare with your  financial institution's records.

Explanation:

The three statements above represent ways that can be used to avoid spending more than what is in one's bank account.

Cheking one's bank statement once a month is the most effective method: by doing so, one can get informed about the exact amount of money left in the account.

Asking one's financial institution to notify you when you are close to $0 is also a good measure, although it could be dangerous to depend on that warning in case one has a tendency to overspend, and the notification comes too early in the month.

Finally, keeping personal records to compare with the records of the bank is more like a learning excercise, that also helps, but it is less effective because it is the bank record that will be right all of the time.

u = 30

continuing your method

= +

subtract from both sides

= - thus

= -

= ⇒ u = 30