You do need a calculator for this.
Three couples; Shem, Ham, and Japheth, and their wives each produce one child per year for 10 years. When the youngest reaches the marriageable age of 20, a full breeding cycle of 30 years begins. At end of the first 30 years the population is 30, a multiple of 5x the original number of people.
Year 1= 6 people.
Year 30....30 people, or 5x growth each 30 year cycle. This multiplier remains constant for each breeding cycle thereafter, so,
Year 60=150 people
Year 90=750
Year110=3750
Year140=18750
Year 170=93750
Year 200=468,750
Year 230=2,343,750
Year 260=11,187,750
Year 290=58,593,750
and so on.
These numbers are also cumulative as the preceding three generations would still be alive if 90 years was the average age of death. So in my calculations the actual population 290 years after the flood would be,
2,343,750
11,187,750
58,593,750
72,125,250
Problem solved.