Excel “Protect” feature

I work with a lot of spreadsheets making bids for clients, so there are times when I want to protect my work and I assign a password to the spreadsheet. I was showing this to my daughter and she said ” I can get around the password!

She copied the spreadsheet , with the button to the left of the A column, pasted it to a new spreadsheet, changed some numbers in the spreadsheet, named it the same thing and wala, she changed my profit % and I would have never caught it. Is there a way to keep this from happening?