How to add 20% of buy token in presale(Crowdsale) if user bought worth of 5 BNB?

Smart Contracts
1

Source of Presale contract:https://github.com/laronlineworld/BYBPresale/blob/main/BYBPresale.sol

/**
* @dev Calculates the token amount per ETH contributed based on the time now.
* @return Rate of amount of GSTAR per Ether as of current time.
*/
function getRate() public view returns (uint256) {
    //calculate bonus based on timing
    if (block.timestamp <= startTime) { return ((rate / 100) * 120); } // 20 percent bonus on presale period, returns 12000
    if (block.timestamp <= startTime.add(1 days)) {return ((rate / 100) * 108);} // 8 percent bonus on day one, return 10800

    return rate;
}

I want to change the rate that instead of timestamp base to earn additional 20%, User need to buy worth of 5 BNB to get additional 20% of token worth bought of 5 BNB

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