CrowdSale.sol

function buyTokens(address _investor) public payable {
    uint256 weiAmount = msg.value;
    _preValidatePurchase(_investor, weiAmount);
    
    
    // update state
    weiRaised = weiRaised.add(weiAmount);
    
    // Calculate the token amount
     uint256 tokens = _calculateToken(weiAmount);
    
    // Transfer them to the investor address
     _processPurchase(_investor, tokens);
}

function _deliverTokens(address _investor, uint256 _tokenAmount) internal {
    token.transfer(_investor, _tokenAmount);
}

function _processPurchase(address _investor, uint256 _tokenAmount)
    internal
    {
      _deliverTokens(_investor, _tokenAmount);
    }

function _calculateToken(uint256 _weiAmount)
    internal
    view
    returns (uint256)
    {
      return _weiAmount.mul(rate);
    }