Let's keep it simple.
Person A invests 1 BTC during day 1-2;
Person B invests 1 BTC during day 3-4;
Person C invests 1 BTC during day 5-6.
Total actual BTC raised is 3btc. But before shares are calculated you have to apply the relevant multiples.
Day 1+2 BTC multiplied with 1.25
Day 3+4 BTC multiplied with 1.1
Day 5+6 BTC multiplied with 1.0
Day 1-2 = 1.25btc;
day 3-4 = 1.1btc;
day 5-6 = 1btc. Total = 3.35.
Divide 68million by 3.35 = 20,298,507.462686 trezarcoins. If you do not apply the multiplyer when distributing coins person A who invested 1 BTC on days 1-2 would only get 20,298,507.462686. Person B and C would also get the same amount and the total distribution would be below 68million.
So you have to apply the multiplyer again to calculate how many coins each person should receive. When you do this person A now gets 25,373,134.328358 person B gets 22,328,358.208955 and person C gets 20,298,507.462686. The sum of all parts is 68 million.