Fill in the missing number. There are blank months in one year and nine months.
If one year and nine months have passed, how long has passed? One year plus nine months, we can’t really add year and month together because they have different units. They’re different measurements of time. We need to convert this one year into months before we can add. And that requires us to know how many months are in a year. One year contains 12 months.
Now that we have 12 months plus nine months, we can add them together. We do that by adding 12 plus nine. 12 plus nine equals 21. And that means 12 months plus nine months equals 21 months. There are 21 months in one year and nine months.