@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 **592nd Year** of Noah. Noah began preaching fromnhis **480th Year** up to his **600th Year** since God gave mankind **120 Years** to repent.