If you have a value with a .5, it means one-half of that value.
i.e. 5/10 = 1/2
If you have a value with a .25, it means one-quarter of that value.
i.e. 25/100 = 1/4
We have a value with .1 here, and it means:
1/10 of a month
In a month, there are 30 days average.
1/10 of 30 days is 3 days.
Therefore, 30.1 months means 30 months and 3 days
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