Amma hikes 2and3/4 miles of nature trail in 1 hour and 15 minutes. how many miles does amman hike per hour?

Formula

d = r * t

Givens

d = 2 3/4
t = 1 hour 15 minutes 
r =

Calculations

You need to convert the 15 minutes to hours.
1 hour = 60 minutes
x hour = 15 minutes

1/60 = x/15
x = 15/60
x = 0.25 hours
so the time = 1 hours + 0.25 hours
t = 1.25 hours

Now you need to do something with the distance.
3/4 mile = 0.75 of a mile
So the distance = 2 + 0.75 = 2.75 miles

Now you are ready for the answer.
d = r * t you are looking for r
2.75 = r * 1.25
r = 2.75/1.25
r = 2.2 miles / hour

Note you could do this with fractions.
2 3/4 = 11/4
1 1/4 =  5/4

r = 11/4 // 5/4 =11/5 = 2 1/5 which is the same thing as 2.2. If your calculator can handle fractions (those kind of calculators cost about 10 dollars in Canada. It should be 8 in the US) that might be the way to do this problem.