@Ozzie Ozzie has it correct! If you do the math and assume healthy births for all three sons...
- 3 Children × 9 Months = 27 Months
- 27 Months ÷ 12 Months = 2.25 Years/2 Years 3 Months
- 2 Children × 9 Months = 18 Months
- 18 Months ÷ 12 Months = 1.5 Years/1 Year 6 Months
This means Japheth would've been 9 Months at Shem's birth, and 1 Year 6 Months old at Ham's birth. And Shem would be 9 Months at Ham's birth. And since Shem was 100 Years old two years after the Flood, he was 98 Years old when it began in the 600th Year of Noah: therefore Shem was born in the 502nd Year of Noah. Noah began preaching fromnhis 480th Year up to his 600th Year since God gave mankind 120 Years to repent.