Question

How many days are in 7.5 years

Answer 1

You can't use calendars to answer this. 

We put different numbers of days in different calendars, to account for the
fraction of a day in the Earth's trip around the sun.  So if you used calendars
to answer the question, the answer would depend on what year, or what part
of a year, you start counting in. There could be either one or two Leap years
in the period you choose.

The only honest and effective way to calculate the answer is to take the word "year"
to mean the real length of the Earth's orbital trip.

The length of the Earth's sidereal year is 365.2536 mean solar days. (rounded)

No matter what year you start counting, or what part of what day of what year,
in 7.5 sidereal years you'll count 2,739 mean solar days, and then the count will
end roughly another 9 hours and 38 minutes later. 

Answer 2

In one year, there is 365 days, and every 4th year it's 366.
in 7.5 years there is only one 4 year, so 1 year with 366 years.

1 year = 365 days
7.5 years = x days

x = 7.5 * 365
x = 2737.5 days
and since we have a leap year of 366 days, we get
 2737.5 + 1 = 2738.5

So, there is 2738.5 days in 7.5 years

Hope this Helps :)