Add balance function wipes out the previous balance. [Step 8 - withdraw, transfer]

[srapper142]

Just going through the tutorial and noticed that when adding funds to an account with an existing balance, the new amount replaces the old amount. In a banking model I would expect that after adding funds to the account the new balance would be the original plus the amount added.

Tutorial Step 8 -
https://openquest.xyz/?github_url=https://raw.githubusercontent.com/CreatorOS/Building-a-bank-with-Solidity-that-isnt-a-toy-For-beginners/main
withdraw, transfer uses sample code https://remix.ethereum.org/#version=soljson-v0.8.4+commit.c7e474f2.js&optimize=false&runs=200&gist=a89ad8b401426cb74656979d6dd6e58a&evmVersion=null

subquest-8b.sol

   function addBalance() public payable {
        balances[msg.sender] = msg.value;
        totalContractBalance = totalContractBalance + msg.value;
        depositTimestamps[msg.sender] = block.timestamp;
    }

I think modified as follows makes more sense,

    function addBalance() public payable {
+       balances[msg.sender] = balances[msg.sender] + msg.value;
-        balances[msg.sender] = msg.value;
        totalContractBalance = totalContractBalance + msg.value;
        depositTimestamps[msg.sender] = block.timestamp;
    }