Madison is training for a marathon. her goal is to run 26.2 miles a day. she currently can run 18.5 miles in a day. about how many more miles does she need to run in a day to meet her goal?

So,

This is simply subtraction.

26.2 - 18.5 = 7.7 mi.
This is exact.

However, if you wanted to round to the nearest mile, it would look like this:
26 - 19 = 7 mi.
This is because in 26.2 mi., 2 is less than 5, and in 18.5 mi., 5 is greater than or equal to 5.
This is rounding to the nearest mile.

Let's roound this
26.2 rounds to 26
18.5 rounds to 19 since 5 or more rounds up
so she runs 19 and wants to run 26, estimate differnce
26-19=7

the answer is 7 miles