wipeout of Staking2Attack.sol in Challenge 5 is puzzling

[erosjohn]

Taking the situation of feeExempt as an example:
wipeout = uint248((tokenBalance * rewardBalance + lastReward * stake) / (stake - tokenBalance))
The above expression is equivalent to:
(wipeout + rewardBalance) = (wipeout - lastReward) * stake / tokenBalance
The left side of the equation represents the total reward(including we added) we need to receive, which is correct.
But there seems to be a issue on the right side of the equation, it should be the actual reward through calling sendReward function.
It is calculated through the following code:

amount = _tokenInfo.totalReward - _stakerInfo.lastReward; // line-1
if (balance != 0) {
    amount = (amount * staked) / balance; 
}

As line-1 shows, actually wipeout shouldn't be _TokenInfo.totalReward, which should be _TokenInfo.totalReward -_ StacerInfo.lastReward. _tokenInfo.totalReward - _stakerInfo.lastReward means the value added by calling function addReward, which is wipeout.
So I think the correct equation may be (wipeout + rewardBalance) = wipeout * stake / tokenBalance, which can be used to derive the actual expression of wideout as (tokenBalance * rewardBalance) / (staked - tokenBalance).
Of course, it's also possible that I misunderstood. Please help me correct it.